1// SPDX-License-Identifier: GPL-2.0
   2/* kernel/rwsem.c: R/W semaphores, public implementation
   3 *
   4 * Written by David Howells (dhowells@redhat.com).
   5 * Derived from asm-i386/semaphore.h
   6 *
   7 * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
   8 * and Michel Lespinasse <walken@google.com>
   9 *
  10 * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
  11 * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
  12 *
  13 * Rwsem count bit fields re-definition and rwsem rearchitecture by
  14 * Waiman Long <longman@redhat.com> and
  15 * Peter Zijlstra <peterz@infradead.org>.
  16 */
  17
  18#include <linux/types.h>
  19#include <linux/kernel.h>
  20#include <linux/sched.h>
  21#include <linux/sched/rt.h>
  22#include <linux/sched/task.h>
  23#include <linux/sched/debug.h>
  24#include <linux/sched/wake_q.h>
  25#include <linux/sched/signal.h>
  26#include <linux/sched/clock.h>
  27#include <linux/export.h>
  28#include <linux/rwsem.h>
  29#include <linux/atomic.h>
  30#include <trace/events/lock.h>
  31
  32#ifndef CONFIG_PREEMPT_RT
  33#include "lock_events.h"
  34
  35/*
  36 * The least significant 2 bits of the owner value has the following
  37 * meanings when set.
  38 *  - Bit 0: RWSEM_READER_OWNED - rwsem may be owned by readers (just a hint)
  39 *  - Bit 1: RWSEM_NONSPINNABLE - Cannot spin on a reader-owned lock
  40 *
  41 * When the rwsem is reader-owned and a spinning writer has timed out,
  42 * the nonspinnable bit will be set to disable optimistic spinning.
 
 
 
 
 
 
  43
  44 * When a writer acquires a rwsem, it puts its task_struct pointer
  45 * into the owner field. It is cleared after an unlock.
  46 *
  47 * When a reader acquires a rwsem, it will also puts its task_struct
  48 * pointer into the owner field with the RWSEM_READER_OWNED bit set.
  49 * On unlock, the owner field will largely be left untouched. So
  50 * for a free or reader-owned rwsem, the owner value may contain
  51 * information about the last reader that acquires the rwsem.
  52 *
  53 * That information may be helpful in debugging cases where the system
  54 * seems to hang on a reader owned rwsem especially if only one reader
  55 * is involved. Ideally we would like to track all the readers that own
  56 * a rwsem, but the overhead is simply too big.
  57 *
  58 * A fast path reader optimistic lock stealing is supported when the rwsem
  59 * is previously owned by a writer and the following conditions are met:
  60 *  - rwsem is not currently writer owned
  61 *  - the handoff isn't set.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  62 */
  63#define RWSEM_READER_OWNED	(1UL << 0)
  64#define RWSEM_NONSPINNABLE	(1UL << 1)
 
 
  65#define RWSEM_OWNER_FLAGS_MASK	(RWSEM_READER_OWNED | RWSEM_NONSPINNABLE)
  66
  67#ifdef CONFIG_DEBUG_RWSEMS
  68# define DEBUG_RWSEMS_WARN_ON(c, sem)	do {			\
  69	if (!debug_locks_silent &&				\
  70	    WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, magic = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
  71		#c, atomic_long_read(&(sem)->count),		\
  72		(unsigned long) sem->magic,			\
  73		atomic_long_read(&(sem)->owner), (long)current,	\
  74		list_empty(&(sem)->wait_list) ? "" : "not "))	\
  75			debug_locks_off();			\
  76	} while (0)
  77#else
  78# define DEBUG_RWSEMS_WARN_ON(c, sem)
  79#endif
  80
  81/*
  82 * On 64-bit architectures, the bit definitions of the count are:
  83 *
  84 * Bit  0    - writer locked bit
  85 * Bit  1    - waiters present bit
  86 * Bit  2    - lock handoff bit
  87 * Bits 3-7  - reserved
  88 * Bits 8-62 - 55-bit reader count
  89 * Bit  63   - read fail bit
  90 *
  91 * On 32-bit architectures, the bit definitions of the count are:
  92 *
  93 * Bit  0    - writer locked bit
  94 * Bit  1    - waiters present bit
  95 * Bit  2    - lock handoff bit
  96 * Bits 3-7  - reserved
  97 * Bits 8-30 - 23-bit reader count
  98 * Bit  31   - read fail bit
  99 *
 100 * It is not likely that the most significant bit (read fail bit) will ever
 101 * be set. This guard bit is still checked anyway in the down_read() fastpath
 102 * just in case we need to use up more of the reader bits for other purpose
 103 * in the future.
 104 *
 105 * atomic_long_fetch_add() is used to obtain reader lock, whereas
 106 * atomic_long_cmpxchg() will be used to obtain writer lock.
 107 *
 108 * There are three places where the lock handoff bit may be set or cleared.
 109 * 1) rwsem_mark_wake() for readers		-- set, clear
 110 * 2) rwsem_try_write_lock() for writers	-- set, clear
 111 * 3) rwsem_del_waiter()			-- clear
 112 *
 113 * For all the above cases, wait_lock will be held. A writer must also
 114 * be the first one in the wait_list to be eligible for setting the handoff
 115 * bit. So concurrent setting/clearing of handoff bit is not possible.
 116 */
 117#define RWSEM_WRITER_LOCKED	(1UL << 0)
 118#define RWSEM_FLAG_WAITERS	(1UL << 1)
 119#define RWSEM_FLAG_HANDOFF	(1UL << 2)
 120#define RWSEM_FLAG_READFAIL	(1UL << (BITS_PER_LONG - 1))
 121
 122#define RWSEM_READER_SHIFT	8
 123#define RWSEM_READER_BIAS	(1UL << RWSEM_READER_SHIFT)
 124#define RWSEM_READER_MASK	(~(RWSEM_READER_BIAS - 1))
 125#define RWSEM_WRITER_MASK	RWSEM_WRITER_LOCKED
 126#define RWSEM_LOCK_MASK		(RWSEM_WRITER_MASK|RWSEM_READER_MASK)
 127#define RWSEM_READ_FAILED_MASK	(RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
 128				 RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)
 129
 130/*
 131 * All writes to owner are protected by WRITE_ONCE() to make sure that
 132 * store tearing can't happen as optimistic spinners may read and use
 133 * the owner value concurrently without lock. Read from owner, however,
 134 * may not need READ_ONCE() as long as the pointer value is only used
 135 * for comparison and isn't being dereferenced.
 136 *
 137 * Both rwsem_{set,clear}_owner() functions should be in the same
 138 * preempt disable section as the atomic op that changes sem->count.
 139 */
 140static inline void rwsem_set_owner(struct rw_semaphore *sem)
 141{
 142	lockdep_assert_preemption_disabled();
 143	atomic_long_set(&sem->owner, (long)current);
 144}
 145
 146static inline void rwsem_clear_owner(struct rw_semaphore *sem)
 147{
 148	lockdep_assert_preemption_disabled();
 149	atomic_long_set(&sem->owner, 0);
 150}
 151
 152/*
 153 * Test the flags in the owner field.
 154 */
 155static inline bool rwsem_test_oflags(struct rw_semaphore *sem, long flags)
 156{
 157	return atomic_long_read(&sem->owner) & flags;
 158}
 159
 160/*
 161 * The task_struct pointer of the last owning reader will be left in
 162 * the owner field.
 163 *
 164 * Note that the owner value just indicates the task has owned the rwsem
 165 * previously, it may not be the real owner or one of the real owners
 166 * anymore when that field is examined, so take it with a grain of salt.
 167 *
 168 * The reader non-spinnable bit is preserved.
 169 */
 170static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
 171					    struct task_struct *owner)
 172{
 173	unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
 174		(atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE);
 175
 176	atomic_long_set(&sem->owner, val);
 177}
 178
 179static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
 180{
 181	__rwsem_set_reader_owned(sem, current);
 182}
 183
 184/*
 185 * Return true if the rwsem is owned by a reader.
 186 */
 187static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
 188{
 189#ifdef CONFIG_DEBUG_RWSEMS
 190	/*
 191	 * Check the count to see if it is write-locked.
 192	 */
 193	long count = atomic_long_read(&sem->count);
 194
 195	if (count & RWSEM_WRITER_MASK)
 196		return false;
 197#endif
 198	return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
 199}
 200
 201#ifdef CONFIG_DEBUG_RWSEMS
 202/*
 203 * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
 204 * is a task pointer in owner of a reader-owned rwsem, it will be the
 205 * real owner or one of the real owners. The only exception is when the
 206 * unlock is done by up_read_non_owner().
 207 */
 208static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
 209{
 210	unsigned long val = atomic_long_read(&sem->owner);
 211
 212	while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) {
 213		if (atomic_long_try_cmpxchg(&sem->owner, &val,
 214					    val & RWSEM_OWNER_FLAGS_MASK))
 215			return;
 216	}
 217}
 218#else
 219static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
 220{
 221}
 222#endif
 223
 224/*
 225 * Set the RWSEM_NONSPINNABLE bits if the RWSEM_READER_OWNED flag
 226 * remains set. Otherwise, the operation will be aborted.
 227 */
 228static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
 229{
 230	unsigned long owner = atomic_long_read(&sem->owner);
 231
 232	do {
 233		if (!(owner & RWSEM_READER_OWNED))
 234			break;
 235		if (owner & RWSEM_NONSPINNABLE)
 236			break;
 237	} while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
 238					  owner | RWSEM_NONSPINNABLE));
 239}
 240
 241static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cntp)
 242{
 243	*cntp = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
 244
 245	if (WARN_ON_ONCE(*cntp < 0))
 246		rwsem_set_nonspinnable(sem);
 247
 248	if (!(*cntp & RWSEM_READ_FAILED_MASK)) {
 249		rwsem_set_reader_owned(sem);
 250		return true;
 251	}
 252
 253	return false;
 254}
 255
 256static inline bool rwsem_write_trylock(struct rw_semaphore *sem)
 257{
 258	long tmp = RWSEM_UNLOCKED_VALUE;
 259
 260	if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, RWSEM_WRITER_LOCKED)) {
 261		rwsem_set_owner(sem);
 262		return true;
 263	}
 264
 265	return false;
 266}
 267
 268/*
 269 * Return just the real task structure pointer of the owner
 270 */
 271static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem)
 272{
 273	return (struct task_struct *)
 274		(atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK);
 275}
 276
 277/*
 278 * Return the real task structure pointer of the owner and the embedded
 279 * flags in the owner. pflags must be non-NULL.
 280 */
 281static inline struct task_struct *
 282rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags)
 283{
 284	unsigned long owner = atomic_long_read(&sem->owner);
 285
 286	*pflags = owner & RWSEM_OWNER_FLAGS_MASK;
 287	return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK);
 288}
 289
 290/*
 291 * Guide to the rw_semaphore's count field.
 292 *
 293 * When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
 294 * by a writer.
 295 *
 296 * The lock is owned by readers when
 297 * (1) the RWSEM_WRITER_LOCKED isn't set in count,
 298 * (2) some of the reader bits are set in count, and
 299 * (3) the owner field has RWSEM_READ_OWNED bit set.
 300 *
 301 * Having some reader bits set is not enough to guarantee a readers owned
 302 * lock as the readers may be in the process of backing out from the count
 303 * and a writer has just released the lock. So another writer may steal
 304 * the lock immediately after that.
 305 */
 306
 307/*
 308 * Initialize an rwsem:
 309 */
 310void __init_rwsem(struct rw_semaphore *sem, const char *name,
 311		  struct lock_class_key *key)
 312{
 313#ifdef CONFIG_DEBUG_LOCK_ALLOC
 314	/*
 315	 * Make sure we are not reinitializing a held semaphore:
 316	 */
 317	debug_check_no_locks_freed((void *)sem, sizeof(*sem));
 318	lockdep_init_map_wait(&sem->dep_map, name, key, 0, LD_WAIT_SLEEP);
 319#endif
 320#ifdef CONFIG_DEBUG_RWSEMS
 321	sem->magic = sem;
 322#endif
 323	atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
 324	raw_spin_lock_init(&sem->wait_lock);
 325	INIT_LIST_HEAD(&sem->wait_list);
 326	atomic_long_set(&sem->owner, 0L);
 327#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 328	osq_lock_init(&sem->osq);
 329#endif
 330}
 331EXPORT_SYMBOL(__init_rwsem);
 332
 333enum rwsem_waiter_type {
 334	RWSEM_WAITING_FOR_WRITE,
 335	RWSEM_WAITING_FOR_READ
 336};
 337
 338struct rwsem_waiter {
 339	struct list_head list;
 340	struct task_struct *task;
 341	enum rwsem_waiter_type type;
 342	unsigned long timeout;
 343	bool handoff_set;
 344};
 345#define rwsem_first_waiter(sem) \
 346	list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
 347
 348enum rwsem_wake_type {
 349	RWSEM_WAKE_ANY,		/* Wake whatever's at head of wait list */
 350	RWSEM_WAKE_READERS,	/* Wake readers only */
 351	RWSEM_WAKE_READ_OWNED	/* Waker thread holds the read lock */
 352};
 353
 
 
 
 
 
 
 354/*
 355 * The typical HZ value is either 250 or 1000. So set the minimum waiting
 356 * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
 357 * queue before initiating the handoff protocol.
 358 */
 359#define RWSEM_WAIT_TIMEOUT	DIV_ROUND_UP(HZ, 250)
 360
 361/*
 362 * Magic number to batch-wakeup waiting readers, even when writers are
 363 * also present in the queue. This both limits the amount of work the
 364 * waking thread must do and also prevents any potential counter overflow,
 365 * however unlikely.
 366 */
 367#define MAX_READERS_WAKEUP	0x100
 368
 369static inline void
 370rwsem_add_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
 371{
 372	lockdep_assert_held(&sem->wait_lock);
 373	list_add_tail(&waiter->list, &sem->wait_list);
 374	/* caller will set RWSEM_FLAG_WAITERS */
 375}
 376
 377/*
 378 * Remove a waiter from the wait_list and clear flags.
 379 *
 380 * Both rwsem_mark_wake() and rwsem_try_write_lock() contain a full 'copy' of
 381 * this function. Modify with care.
 382 *
 383 * Return: true if wait_list isn't empty and false otherwise
 384 */
 385static inline bool
 386rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
 387{
 388	lockdep_assert_held(&sem->wait_lock);
 389	list_del(&waiter->list);
 390	if (likely(!list_empty(&sem->wait_list)))
 391		return true;
 392
 393	atomic_long_andnot(RWSEM_FLAG_HANDOFF | RWSEM_FLAG_WAITERS, &sem->count);
 394	return false;
 395}
 396
 397/*
 398 * handle the lock release when processes blocked on it that can now run
 399 * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
 400 *   have been set.
 401 * - there must be someone on the queue
 402 * - the wait_lock must be held by the caller
 403 * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
 404 *   to actually wakeup the blocked task(s) and drop the reference count,
 405 *   preferably when the wait_lock is released
 406 * - woken process blocks are discarded from the list after having task zeroed
 407 * - writers are only marked woken if downgrading is false
 408 *
 409 * Implies rwsem_del_waiter() for all woken readers.
 410 */
 411static void rwsem_mark_wake(struct rw_semaphore *sem,
 412			    enum rwsem_wake_type wake_type,
 413			    struct wake_q_head *wake_q)
 414{
 415	struct rwsem_waiter *waiter, *tmp;
 416	long oldcount, woken = 0, adjustment = 0;
 417	struct list_head wlist;
 418
 419	lockdep_assert_held(&sem->wait_lock);
 420
 421	/*
 422	 * Take a peek at the queue head waiter such that we can determine
 423	 * the wakeup(s) to perform.
 424	 */
 425	waiter = rwsem_first_waiter(sem);
 426
 427	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
 428		if (wake_type == RWSEM_WAKE_ANY) {
 429			/*
 430			 * Mark writer at the front of the queue for wakeup.
 431			 * Until the task is actually later awoken later by
 432			 * the caller, other writers are able to steal it.
 433			 * Readers, on the other hand, will block as they
 434			 * will notice the queued writer.
 435			 */
 436			wake_q_add(wake_q, waiter->task);
 437			lockevent_inc(rwsem_wake_writer);
 438		}
 439
 440		return;
 441	}
 442
 443	/*
 444	 * No reader wakeup if there are too many of them already.
 445	 */
 446	if (unlikely(atomic_long_read(&sem->count) < 0))
 447		return;
 448
 449	/*
 450	 * Writers might steal the lock before we grant it to the next reader.
 451	 * We prefer to do the first reader grant before counting readers
 452	 * so we can bail out early if a writer stole the lock.
 453	 */
 454	if (wake_type != RWSEM_WAKE_READ_OWNED) {
 455		struct task_struct *owner;
 456
 457		adjustment = RWSEM_READER_BIAS;
 458		oldcount = atomic_long_fetch_add(adjustment, &sem->count);
 459		if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
 460			/*
 461			 * When we've been waiting "too" long (for writers
 462			 * to give up the lock), request a HANDOFF to
 463			 * force the issue.
 464			 */
 465			if (time_after(jiffies, waiter->timeout)) {
 466				if (!(oldcount & RWSEM_FLAG_HANDOFF)) {
 467					adjustment -= RWSEM_FLAG_HANDOFF;
 468					lockevent_inc(rwsem_rlock_handoff);
 469				}
 470				waiter->handoff_set = true;
 471			}
 472
 473			atomic_long_add(-adjustment, &sem->count);
 474			return;
 475		}
 476		/*
 477		 * Set it to reader-owned to give spinners an early
 478		 * indication that readers now have the lock.
 479		 * The reader nonspinnable bit seen at slowpath entry of
 480		 * the reader is copied over.
 481		 */
 482		owner = waiter->task;
 
 
 
 
 483		__rwsem_set_reader_owned(sem, owner);
 484	}
 485
 486	/*
 487	 * Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
 488	 * queue. We know that the woken will be at least 1 as we accounted
 489	 * for above. Note we increment the 'active part' of the count by the
 490	 * number of readers before waking any processes up.
 491	 *
 492	 * This is an adaptation of the phase-fair R/W locks where at the
 493	 * reader phase (first waiter is a reader), all readers are eligible
 494	 * to acquire the lock at the same time irrespective of their order
 495	 * in the queue. The writers acquire the lock according to their
 496	 * order in the queue.
 497	 *
 498	 * We have to do wakeup in 2 passes to prevent the possibility that
 499	 * the reader count may be decremented before it is incremented. It
 500	 * is because the to-be-woken waiter may not have slept yet. So it
 501	 * may see waiter->task got cleared, finish its critical section and
 502	 * do an unlock before the reader count increment.
 503	 *
 504	 * 1) Collect the read-waiters in a separate list, count them and
 505	 *    fully increment the reader count in rwsem.
 506	 * 2) For each waiters in the new list, clear waiter->task and
 507	 *    put them into wake_q to be woken up later.
 508	 */
 509	INIT_LIST_HEAD(&wlist);
 510	list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
 511		if (waiter->type == RWSEM_WAITING_FOR_WRITE)
 512			continue;
 513
 514		woken++;
 515		list_move_tail(&waiter->list, &wlist);
 516
 517		/*
 518		 * Limit # of readers that can be woken up per wakeup call.
 519		 */
 520		if (unlikely(woken >= MAX_READERS_WAKEUP))
 521			break;
 522	}
 523
 524	adjustment = woken * RWSEM_READER_BIAS - adjustment;
 525	lockevent_cond_inc(rwsem_wake_reader, woken);
 526
 527	oldcount = atomic_long_read(&sem->count);
 528	if (list_empty(&sem->wait_list)) {
 529		/*
 530		 * Combined with list_move_tail() above, this implies
 531		 * rwsem_del_waiter().
 532		 */
 533		adjustment -= RWSEM_FLAG_WAITERS;
 534		if (oldcount & RWSEM_FLAG_HANDOFF)
 535			adjustment -= RWSEM_FLAG_HANDOFF;
 536	} else if (woken) {
 537		/*
 538		 * When we've woken a reader, we no longer need to force
 539		 * writers to give up the lock and we can clear HANDOFF.
 540		 */
 541		if (oldcount & RWSEM_FLAG_HANDOFF)
 542			adjustment -= RWSEM_FLAG_HANDOFF;
 543	}
 544
 
 
 
 
 
 
 
 545	if (adjustment)
 546		atomic_long_add(adjustment, &sem->count);
 547
 548	/* 2nd pass */
 549	list_for_each_entry_safe(waiter, tmp, &wlist, list) {
 550		struct task_struct *tsk;
 551
 552		tsk = waiter->task;
 553		get_task_struct(tsk);
 554
 555		/*
 556		 * Ensure calling get_task_struct() before setting the reader
 557		 * waiter to nil such that rwsem_down_read_slowpath() cannot
 558		 * race with do_exit() by always holding a reference count
 559		 * to the task to wakeup.
 560		 */
 561		smp_store_release(&waiter->task, NULL);
 562		/*
 563		 * Ensure issuing the wakeup (either by us or someone else)
 564		 * after setting the reader waiter to nil.
 565		 */
 566		wake_q_add_safe(wake_q, tsk);
 567	}
 568}
 569
 570/*
 571 * Remove a waiter and try to wake up other waiters in the wait queue
 572 * This function is called from the out_nolock path of both the reader and
 573 * writer slowpaths with wait_lock held. It releases the wait_lock and
 574 * optionally wake up waiters before it returns.
 575 */
 576static inline void
 577rwsem_del_wake_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter,
 578		      struct wake_q_head *wake_q)
 579		      __releases(&sem->wait_lock)
 580{
 581	bool first = rwsem_first_waiter(sem) == waiter;
 582
 583	wake_q_init(wake_q);
 584
 585	/*
 586	 * If the wait_list isn't empty and the waiter to be deleted is
 587	 * the first waiter, we wake up the remaining waiters as they may
 588	 * be eligible to acquire or spin on the lock.
 589	 */
 590	if (rwsem_del_waiter(sem, waiter) && first)
 591		rwsem_mark_wake(sem, RWSEM_WAKE_ANY, wake_q);
 592	raw_spin_unlock_irq(&sem->wait_lock);
 593	if (!wake_q_empty(wake_q))
 594		wake_up_q(wake_q);
 595}
 596
 597/*
 598 * This function must be called with the sem->wait_lock held to prevent
 599 * race conditions between checking the rwsem wait list and setting the
 600 * sem->count accordingly.
 601 *
 602 * Implies rwsem_del_waiter() on success.
 
 603 */
 604static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
 605					struct rwsem_waiter *waiter)
 606{
 607	struct rwsem_waiter *first = rwsem_first_waiter(sem);
 608	long count, new;
 609
 610	lockdep_assert_held(&sem->wait_lock);
 611
 612	count = atomic_long_read(&sem->count);
 613	do {
 614		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
 615
 616		if (has_handoff) {
 617			/*
 618			 * Honor handoff bit and yield only when the first
 619			 * waiter is the one that set it. Otherwisee, we
 620			 * still try to acquire the rwsem.
 621			 */
 622			if (first->handoff_set && (waiter != first))
 623				return false;
 624		}
 625
 626		new = count;
 627
 628		if (count & RWSEM_LOCK_MASK) {
 629			/*
 630			 * A waiter (first or not) can set the handoff bit
 631			 * if it is an RT task or wait in the wait queue
 632			 * for too long.
 633			 */
 634			if (has_handoff || (!rt_task(waiter->task) &&
 635					    !time_after(jiffies, waiter->timeout)))
 636				return false;
 637
 638			new |= RWSEM_FLAG_HANDOFF;
 639		} else {
 640			new |= RWSEM_WRITER_LOCKED;
 641			new &= ~RWSEM_FLAG_HANDOFF;
 642
 643			if (list_is_singular(&sem->wait_list))
 644				new &= ~RWSEM_FLAG_WAITERS;
 645		}
 646	} while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
 647
 648	/*
 649	 * We have either acquired the lock with handoff bit cleared or set
 650	 * the handoff bit. Only the first waiter can have its handoff_set
 651	 * set here to enable optimistic spinning in slowpath loop.
 652	 */
 653	if (new & RWSEM_FLAG_HANDOFF) {
 654		first->handoff_set = true;
 655		lockevent_inc(rwsem_wlock_handoff);
 656		return false;
 657	}
 658
 659	/*
 660	 * Have rwsem_try_write_lock() fully imply rwsem_del_waiter() on
 661	 * success.
 662	 */
 663	list_del(&waiter->list);
 664	rwsem_set_owner(sem);
 665	return true;
 666}
 667
 
 668/*
 669 * The rwsem_spin_on_owner() function returns the following 4 values
 670 * depending on the lock owner state.
 671 *   OWNER_NULL  : owner is currently NULL
 672 *   OWNER_WRITER: when owner changes and is a writer
 673 *   OWNER_READER: when owner changes and the new owner may be a reader.
 674 *   OWNER_NONSPINNABLE:
 675 *		   when optimistic spinning has to stop because either the
 676 *		   owner stops running, is unknown, or its timeslice has
 677 *		   been used up.
 678 */
 679enum owner_state {
 680	OWNER_NULL		= 1 << 0,
 681	OWNER_WRITER		= 1 << 1,
 682	OWNER_READER		= 1 << 2,
 683	OWNER_NONSPINNABLE	= 1 << 3,
 684};
 
 
 
 
 
 
 
 
 
 
 
 
 685
 686#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 687/*
 688 * Try to acquire write lock before the writer has been put on wait queue.
 689 */
 690static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
 691{
 692	long count = atomic_long_read(&sem->count);
 693
 694	while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
 695		if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
 696					count | RWSEM_WRITER_LOCKED)) {
 697			rwsem_set_owner(sem);
 698			lockevent_inc(rwsem_opt_lock);
 699			return true;
 700		}
 701	}
 702	return false;
 703}
 704
 705static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 
 
 
 
 
 
 
 
 
 
 706{
 707	struct task_struct *owner;
 708	unsigned long flags;
 709	bool ret = true;
 710
 711	if (need_resched()) {
 712		lockevent_inc(rwsem_opt_fail);
 713		return false;
 714	}
 715
 716	/*
 717	 * Disable preemption is equal to the RCU read-side crital section,
 718	 * thus the task_strcut structure won't go away.
 719	 */
 720	owner = rwsem_owner_flags(sem, &flags);
 721	/*
 722	 * Don't check the read-owner as the entry may be stale.
 723	 */
 724	if ((flags & RWSEM_NONSPINNABLE) ||
 725	    (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
 726		ret = false;
 
 
 727
 728	lockevent_cond_inc(rwsem_opt_fail, !ret);
 729	return ret;
 730}
 731
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 732#define OWNER_SPINNABLE		(OWNER_NULL | OWNER_WRITER | OWNER_READER)
 733
 734static inline enum owner_state
 735rwsem_owner_state(struct task_struct *owner, unsigned long flags)
 736{
 737	if (flags & RWSEM_NONSPINNABLE)
 738		return OWNER_NONSPINNABLE;
 739
 740	if (flags & RWSEM_READER_OWNED)
 741		return OWNER_READER;
 742
 743	return owner ? OWNER_WRITER : OWNER_NULL;
 744}
 745
 746static noinline enum owner_state
 747rwsem_spin_on_owner(struct rw_semaphore *sem)
 748{
 749	struct task_struct *new, *owner;
 750	unsigned long flags, new_flags;
 751	enum owner_state state;
 752
 753	lockdep_assert_preemption_disabled();
 754
 755	owner = rwsem_owner_flags(sem, &flags);
 756	state = rwsem_owner_state(owner, flags);
 757	if (state != OWNER_WRITER)
 758		return state;
 759
 
 760	for (;;) {
 761		/*
 762		 * When a waiting writer set the handoff flag, it may spin
 763		 * on the owner as well. Once that writer acquires the lock,
 764		 * we can spin on it. So we don't need to quit even when the
 765		 * handoff bit is set.
 766		 */
 767		new = rwsem_owner_flags(sem, &new_flags);
 768		if ((new != owner) || (new_flags != flags)) {
 769			state = rwsem_owner_state(new, new_flags);
 770			break;
 771		}
 772
 773		/*
 774		 * Ensure we emit the owner->on_cpu, dereference _after_
 775		 * checking sem->owner still matches owner, if that fails,
 776		 * owner might point to free()d memory, if it still matches,
 777		 * our spinning context already disabled preemption which is
 778		 * equal to RCU read-side crital section ensures the memory
 779		 * stays valid.
 780		 */
 781		barrier();
 782
 783		if (need_resched() || !owner_on_cpu(owner)) {
 784			state = OWNER_NONSPINNABLE;
 785			break;
 786		}
 787
 788		cpu_relax();
 789	}
 
 790
 791	return state;
 792}
 793
 794/*
 795 * Calculate reader-owned rwsem spinning threshold for writer
 796 *
 797 * The more readers own the rwsem, the longer it will take for them to
 798 * wind down and free the rwsem. So the empirical formula used to
 799 * determine the actual spinning time limit here is:
 800 *
 801 *   Spinning threshold = (10 + nr_readers/2)us
 802 *
 803 * The limit is capped to a maximum of 25us (30 readers). This is just
 804 * a heuristic and is subjected to change in the future.
 805 */
 806static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
 807{
 808	long count = atomic_long_read(&sem->count);
 809	int readers = count >> RWSEM_READER_SHIFT;
 810	u64 delta;
 811
 812	if (readers > 30)
 813		readers = 30;
 814	delta = (20 + readers) * NSEC_PER_USEC / 2;
 815
 816	return sched_clock() + delta;
 817}
 818
 819static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 820{
 821	bool taken = false;
 822	int prev_owner_state = OWNER_NULL;
 823	int loop = 0;
 824	u64 rspin_threshold = 0;
 
 
 
 
 825
 826	/* sem->wait_lock should not be held when doing optimistic spinning */
 827	if (!osq_lock(&sem->osq))
 828		goto done;
 829
 830	/*
 831	 * Optimistically spin on the owner field and attempt to acquire the
 832	 * lock whenever the owner changes. Spinning will be stopped when:
 833	 *  1) the owning writer isn't running; or
 834	 *  2) readers own the lock and spinning time has exceeded limit.
 835	 */
 836	for (;;) {
 837		enum owner_state owner_state;
 838
 839		owner_state = rwsem_spin_on_owner(sem);
 840		if (!(owner_state & OWNER_SPINNABLE))
 841			break;
 842
 843		/*
 844		 * Try to acquire the lock
 845		 */
 846		taken = rwsem_try_write_lock_unqueued(sem);
 
 847
 848		if (taken)
 849			break;
 850
 851		/*
 852		 * Time-based reader-owned rwsem optimistic spinning
 853		 */
 854		if (owner_state == OWNER_READER) {
 855			/*
 856			 * Re-initialize rspin_threshold every time when
 857			 * the owner state changes from non-reader to reader.
 858			 * This allows a writer to steal the lock in between
 859			 * 2 reader phases and have the threshold reset at
 860			 * the beginning of the 2nd reader phase.
 861			 */
 862			if (prev_owner_state != OWNER_READER) {
 863				if (rwsem_test_oflags(sem, RWSEM_NONSPINNABLE))
 864					break;
 865				rspin_threshold = rwsem_rspin_threshold(sem);
 866				loop = 0;
 867			}
 868
 869			/*
 870			 * Check time threshold once every 16 iterations to
 871			 * avoid calling sched_clock() too frequently so
 872			 * as to reduce the average latency between the times
 873			 * when the lock becomes free and when the spinner
 874			 * is ready to do a trylock.
 875			 */
 876			else if (!(++loop & 0xf) && (sched_clock() > rspin_threshold)) {
 877				rwsem_set_nonspinnable(sem);
 878				lockevent_inc(rwsem_opt_nospin);
 879				break;
 880			}
 881		}
 882
 883		/*
 884		 * An RT task cannot do optimistic spinning if it cannot
 885		 * be sure the lock holder is running or live-lock may
 886		 * happen if the current task and the lock holder happen
 887		 * to run in the same CPU. However, aborting optimistic
 888		 * spinning while a NULL owner is detected may miss some
 889		 * opportunity where spinning can continue without causing
 890		 * problem.
 891		 *
 892		 * There are 2 possible cases where an RT task may be able
 893		 * to continue spinning.
 894		 *
 895		 * 1) The lock owner is in the process of releasing the
 896		 *    lock, sem->owner is cleared but the lock has not
 897		 *    been released yet.
 898		 * 2) The lock was free and owner cleared, but another
 899		 *    task just comes in and acquire the lock before
 900		 *    we try to get it. The new owner may be a spinnable
 901		 *    writer.
 902		 *
 903		 * To take advantage of two scenarios listed above, the RT
 904		 * task is made to retry one more time to see if it can
 905		 * acquire the lock or continue spinning on the new owning
 906		 * writer. Of course, if the time lag is long enough or the
 907		 * new owner is not a writer or spinnable, the RT task will
 908		 * quit spinning.
 909		 *
 910		 * If the owner is a writer, the need_resched() check is
 911		 * done inside rwsem_spin_on_owner(). If the owner is not
 912		 * a writer, need_resched() check needs to be done here.
 913		 */
 914		if (owner_state != OWNER_WRITER) {
 915			if (need_resched())
 916				break;
 917			if (rt_task(current) &&
 918			   (prev_owner_state != OWNER_WRITER))
 919				break;
 920		}
 921		prev_owner_state = owner_state;
 922
 923		/*
 924		 * The cpu_relax() call is a compiler barrier which forces
 925		 * everything in this loop to be re-loaded. We don't need
 926		 * memory barriers as we'll eventually observe the right
 927		 * values at the cost of a few extra spins.
 928		 */
 929		cpu_relax();
 930	}
 931	osq_unlock(&sem->osq);
 932done:
 
 933	lockevent_cond_inc(rwsem_opt_fail, !taken);
 934	return taken;
 935}
 936
 937/*
 938 * Clear the owner's RWSEM_NONSPINNABLE bit if it is set. This should
 939 * only be called when the reader count reaches 0.
 
 
 
 
 
 
 940 */
 941static inline void clear_nonspinnable(struct rw_semaphore *sem)
 942{
 943	if (unlikely(rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)))
 944		atomic_long_andnot(RWSEM_NONSPINNABLE, &sem->owner);
 945}
 946
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 947#else
 948static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 
 949{
 950	return false;
 951}
 952
 953static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 954{
 955	return false;
 956}
 957
 958static inline void clear_nonspinnable(struct rw_semaphore *sem) { }
 959
 960static inline enum owner_state
 961rwsem_spin_on_owner(struct rw_semaphore *sem)
 962{
 963	return OWNER_NONSPINNABLE;
 964}
 965#endif
 966
 967/*
 968 * Prepare to wake up waiter(s) in the wait queue by putting them into the
 969 * given wake_q if the rwsem lock owner isn't a writer. If rwsem is likely
 970 * reader-owned, wake up read lock waiters in queue front or wake up any
 971 * front waiter otherwise.
 972
 973 * This is being called from both reader and writer slow paths.
 974 */
 975static inline void rwsem_cond_wake_waiter(struct rw_semaphore *sem, long count,
 976					  struct wake_q_head *wake_q)
 977{
 978	enum rwsem_wake_type wake_type;
 979
 980	if (count & RWSEM_WRITER_MASK)
 981		return;
 982
 983	if (count & RWSEM_READER_MASK) {
 984		wake_type = RWSEM_WAKE_READERS;
 985	} else {
 986		wake_type = RWSEM_WAKE_ANY;
 987		clear_nonspinnable(sem);
 988	}
 989	rwsem_mark_wake(sem, wake_type, wake_q);
 990}
 
 
 991
 992/*
 993 * Wait for the read lock to be granted
 994 */
 995static struct rw_semaphore __sched *
 996rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int state)
 997{
 998	long adjustment = -RWSEM_READER_BIAS;
 999	long rcnt = (count >> RWSEM_READER_SHIFT);
1000	struct rwsem_waiter waiter;
1001	DEFINE_WAKE_Q(wake_q);
 
1002
1003	/*
1004	 * To prevent a constant stream of readers from starving a sleeping
1005	 * writer, don't attempt optimistic lock stealing if the lock is
1006	 * very likely owned by readers.
1007	 */
1008	if ((atomic_long_read(&sem->owner) & RWSEM_READER_OWNED) &&
1009	    (rcnt > 1) && !(count & RWSEM_WRITER_LOCKED))
 
 
 
1010		goto queue;
1011
1012	/*
1013	 * Reader optimistic lock stealing.
1014	 */
1015	if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF))) {
1016		rwsem_set_reader_owned(sem);
1017		lockevent_inc(rwsem_rlock_steal);
1018
1019		/*
1020		 * Wake up other readers in the wait queue if it is
1021		 * the first reader.
1022		 */
1023		if ((rcnt == 1) && (count & RWSEM_FLAG_WAITERS)) {
1024			raw_spin_lock_irq(&sem->wait_lock);
1025			if (!list_empty(&sem->wait_list))
1026				rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
1027						&wake_q);
1028			raw_spin_unlock_irq(&sem->wait_lock);
1029			wake_up_q(&wake_q);
1030		}
1031		return sem;
 
 
 
1032	}
1033
1034queue:
1035	waiter.task = current;
1036	waiter.type = RWSEM_WAITING_FOR_READ;
1037	waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1038	waiter.handoff_set = false;
1039
1040	raw_spin_lock_irq(&sem->wait_lock);
1041	if (list_empty(&sem->wait_list)) {
1042		/*
1043		 * In case the wait queue is empty and the lock isn't owned
1044		 * by a writer, this reader can exit the slowpath and return
1045		 * immediately as its RWSEM_READER_BIAS has already been set
1046		 * in the count.
1047		 */
1048		if (!(atomic_long_read(&sem->count) & RWSEM_WRITER_MASK)) {
 
1049			/* Provide lock ACQUIRE */
1050			smp_acquire__after_ctrl_dep();
1051			raw_spin_unlock_irq(&sem->wait_lock);
1052			rwsem_set_reader_owned(sem);
1053			lockevent_inc(rwsem_rlock_fast);
1054			return sem;
1055		}
1056		adjustment += RWSEM_FLAG_WAITERS;
1057	}
1058	rwsem_add_waiter(sem, &waiter);
1059
1060	/* we're now waiting on the lock, but no longer actively locking */
1061	count = atomic_long_add_return(adjustment, &sem->count);
1062
1063	rwsem_cond_wake_waiter(sem, count, &wake_q);
1064	raw_spin_unlock_irq(&sem->wait_lock);
1065
1066	if (!wake_q_empty(&wake_q))
1067		wake_up_q(&wake_q);
 
 
 
 
 
 
 
 
 
 
 
1068
1069	trace_contention_begin(sem, LCB_F_READ);
 
1070
1071	/* wait to be given the lock */
1072	for (;;) {
1073		set_current_state(state);
1074		if (!smp_load_acquire(&waiter.task)) {
1075			/* Matches rwsem_mark_wake()'s smp_store_release(). */
1076			break;
1077		}
1078		if (signal_pending_state(state, current)) {
1079			raw_spin_lock_irq(&sem->wait_lock);
1080			if (waiter.task)
1081				goto out_nolock;
1082			raw_spin_unlock_irq(&sem->wait_lock);
1083			/* Ordered by sem->wait_lock against rwsem_mark_wake(). */
1084			break;
1085		}
1086		schedule_preempt_disabled();
1087		lockevent_inc(rwsem_sleep_reader);
1088	}
1089
1090	__set_current_state(TASK_RUNNING);
1091	lockevent_inc(rwsem_rlock);
1092	trace_contention_end(sem, 0);
1093	return sem;
1094
1095out_nolock:
1096	rwsem_del_wake_waiter(sem, &waiter, &wake_q);
 
 
 
 
 
1097	__set_current_state(TASK_RUNNING);
1098	lockevent_inc(rwsem_rlock_fail);
1099	trace_contention_end(sem, -EINTR);
1100	return ERR_PTR(-EINTR);
1101}
1102
1103/*
 
 
 
 
 
 
 
 
 
 
 
 
 
1104 * Wait until we successfully acquire the write lock
1105 */
1106static struct rw_semaphore __sched *
1107rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
1108{
 
 
 
1109	struct rwsem_waiter waiter;
 
1110	DEFINE_WAKE_Q(wake_q);
1111
1112	/* do optimistic spinning and steal lock if possible */
1113	if (rwsem_can_spin_on_owner(sem) && rwsem_optimistic_spin(sem)) {
 
1114		/* rwsem_optimistic_spin() implies ACQUIRE on success */
1115		return sem;
1116	}
1117
1118	/*
 
 
 
 
 
 
 
1119	 * Optimistic spinning failed, proceed to the slowpath
1120	 * and block until we can acquire the sem.
1121	 */
1122	waiter.task = current;
1123	waiter.type = RWSEM_WAITING_FOR_WRITE;
1124	waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1125	waiter.handoff_set = false;
1126
1127	raw_spin_lock_irq(&sem->wait_lock);
1128	rwsem_add_waiter(sem, &waiter);
 
 
 
 
1129
1130	/* we're now waiting on the lock */
1131	if (rwsem_first_waiter(sem) != &waiter) {
1132		rwsem_cond_wake_waiter(sem, atomic_long_read(&sem->count),
1133				       &wake_q);
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1134		if (!wake_q_empty(&wake_q)) {
1135			/*
1136			 * We want to minimize wait_lock hold time especially
1137			 * when a large number of readers are to be woken up.
1138			 */
1139			raw_spin_unlock_irq(&sem->wait_lock);
1140			wake_up_q(&wake_q);
 
1141			raw_spin_lock_irq(&sem->wait_lock);
1142		}
1143	} else {
1144		atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
1145	}
1146
 
1147	/* wait until we successfully acquire the lock */
1148	set_current_state(state);
1149	trace_contention_begin(sem, LCB_F_WRITE);
1150
1151	for (;;) {
1152		if (rwsem_try_write_lock(sem, &waiter)) {
1153			/* rwsem_try_write_lock() implies ACQUIRE on success */
1154			break;
1155		}
1156
1157		raw_spin_unlock_irq(&sem->wait_lock);
1158
1159		if (signal_pending_state(state, current))
1160			goto out_nolock;
1161
1162		/*
1163		 * After setting the handoff bit and failing to acquire
1164		 * the lock, attempt to spin on owner to accelerate lock
1165		 * transfer. If the previous owner is a on-cpu writer and it
1166		 * has just released the lock, OWNER_NULL will be returned.
1167		 * In this case, we attempt to acquire the lock again
1168		 * without sleeping.
1169		 */
1170		if (waiter.handoff_set) {
1171			enum owner_state owner_state;
 
 
 
 
 
 
1172
1173			owner_state = rwsem_spin_on_owner(sem);
1174			if (owner_state == OWNER_NULL)
1175				goto trylock_again;
1176		}
 
 
 
 
 
1177
1178		schedule_preempt_disabled();
1179		lockevent_inc(rwsem_sleep_writer);
1180		set_current_state(state);
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1181trylock_again:
1182		raw_spin_lock_irq(&sem->wait_lock);
1183	}
1184	__set_current_state(TASK_RUNNING);
 
 
1185	raw_spin_unlock_irq(&sem->wait_lock);
1186	lockevent_inc(rwsem_wlock);
1187	trace_contention_end(sem, 0);
1188	return sem;
1189
1190out_nolock:
1191	__set_current_state(TASK_RUNNING);
1192	raw_spin_lock_irq(&sem->wait_lock);
1193	rwsem_del_wake_waiter(sem, &waiter, &wake_q);
 
 
 
 
 
 
 
 
 
 
1194	lockevent_inc(rwsem_wlock_fail);
1195	trace_contention_end(sem, -EINTR);
1196	return ERR_PTR(-EINTR);
1197}
1198
1199/*
1200 * handle waking up a waiter on the semaphore
1201 * - up_read/up_write has decremented the active part of count if we come here
1202 */
1203static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
1204{
1205	unsigned long flags;
1206	DEFINE_WAKE_Q(wake_q);
1207
1208	raw_spin_lock_irqsave(&sem->wait_lock, flags);
1209
1210	if (!list_empty(&sem->wait_list))
1211		rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
1212
1213	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1214	wake_up_q(&wake_q);
1215
1216	return sem;
1217}
1218
1219/*
1220 * downgrade a write lock into a read lock
1221 * - caller incremented waiting part of count and discovered it still negative
1222 * - just wake up any readers at the front of the queue
1223 */
1224static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
1225{
1226	unsigned long flags;
1227	DEFINE_WAKE_Q(wake_q);
1228
1229	raw_spin_lock_irqsave(&sem->wait_lock, flags);
1230
1231	if (!list_empty(&sem->wait_list))
1232		rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
1233
1234	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1235	wake_up_q(&wake_q);
1236
1237	return sem;
1238}
1239
1240/*
1241 * lock for reading
1242 */
1243static __always_inline int __down_read_common(struct rw_semaphore *sem, int state)
1244{
1245	int ret = 0;
1246	long count;
1247
1248	preempt_disable();
1249	if (!rwsem_read_trylock(sem, &count)) {
1250		if (IS_ERR(rwsem_down_read_slowpath(sem, count, state))) {
1251			ret = -EINTR;
1252			goto out;
1253		}
1254		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
 
 
1255	}
1256out:
1257	preempt_enable();
1258	return ret;
1259}
1260
1261static __always_inline void __down_read(struct rw_semaphore *sem)
1262{
1263	__down_read_common(sem, TASK_UNINTERRUPTIBLE);
1264}
1265
1266static __always_inline int __down_read_interruptible(struct rw_semaphore *sem)
1267{
1268	return __down_read_common(sem, TASK_INTERRUPTIBLE);
1269}
1270
1271static __always_inline int __down_read_killable(struct rw_semaphore *sem)
1272{
1273	return __down_read_common(sem, TASK_KILLABLE);
 
 
 
 
 
 
 
1274}
1275
1276static inline int __down_read_trylock(struct rw_semaphore *sem)
1277{
1278	int ret = 0;
1279	long tmp;
1280
1281	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1282
1283	preempt_disable();
1284	tmp = atomic_long_read(&sem->count);
1285	while (!(tmp & RWSEM_READ_FAILED_MASK)) {
 
 
1286		if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1287						    tmp + RWSEM_READER_BIAS)) {
1288			rwsem_set_reader_owned(sem);
1289			ret = 1;
1290			break;
1291		}
1292	}
1293	preempt_enable();
1294	return ret;
1295}
1296
1297/*
1298 * lock for writing
1299 */
1300static inline int __down_write_common(struct rw_semaphore *sem, int state)
1301{
1302	int ret = 0;
1303
1304	preempt_disable();
1305	if (unlikely(!rwsem_write_trylock(sem))) {
1306		if (IS_ERR(rwsem_down_write_slowpath(sem, state)))
1307			ret = -EINTR;
1308	}
1309	preempt_enable();
1310	return ret;
1311}
1312
1313static inline void __down_write(struct rw_semaphore *sem)
1314{
1315	__down_write_common(sem, TASK_UNINTERRUPTIBLE);
 
 
 
 
 
 
1316}
1317
1318static inline int __down_write_killable(struct rw_semaphore *sem)
1319{
1320	return __down_write_common(sem, TASK_KILLABLE);
 
 
 
 
 
 
 
 
 
1321}
1322
1323static inline int __down_write_trylock(struct rw_semaphore *sem)
1324{
1325	int ret;
1326
1327	preempt_disable();
1328	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1329	ret = rwsem_write_trylock(sem);
1330	preempt_enable();
1331
1332	return ret;
 
 
 
 
 
 
1333}
1334
1335/*
1336 * unlock after reading
1337 */
1338static inline void __up_read(struct rw_semaphore *sem)
1339{
1340	long tmp;
1341
1342	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1343	DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1344
1345	preempt_disable();
1346	rwsem_clear_reader_owned(sem);
1347	tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
1348	DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
1349	if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
1350		      RWSEM_FLAG_WAITERS)) {
1351		clear_nonspinnable(sem);
1352		rwsem_wake(sem);
1353	}
1354	preempt_enable();
1355}
1356
1357/*
1358 * unlock after writing
1359 */
1360static inline void __up_write(struct rw_semaphore *sem)
1361{
1362	long tmp;
1363
1364	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1365	/*
1366	 * sem->owner may differ from current if the ownership is transferred
1367	 * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
1368	 */
1369	DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
1370			    !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);
1371
1372	preempt_disable();
1373	rwsem_clear_owner(sem);
1374	tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
1375	if (unlikely(tmp & RWSEM_FLAG_WAITERS))
1376		rwsem_wake(sem);
1377	preempt_enable();
1378}
1379
1380/*
1381 * downgrade write lock to read lock
1382 */
1383static inline void __downgrade_write(struct rw_semaphore *sem)
1384{
1385	long tmp;
1386
1387	/*
1388	 * When downgrading from exclusive to shared ownership,
1389	 * anything inside the write-locked region cannot leak
1390	 * into the read side. In contrast, anything in the
1391	 * read-locked region is ok to be re-ordered into the
1392	 * write side. As such, rely on RELEASE semantics.
1393	 */
1394	DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem);
1395	preempt_disable();
1396	tmp = atomic_long_fetch_add_release(
1397		-RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
1398	rwsem_set_reader_owned(sem);
1399	if (tmp & RWSEM_FLAG_WAITERS)
1400		rwsem_downgrade_wake(sem);
1401	preempt_enable();
1402}
1403
1404#else /* !CONFIG_PREEMPT_RT */
1405
1406#define RT_MUTEX_BUILD_MUTEX
1407#include "rtmutex.c"
1408
1409#define rwbase_set_and_save_current_state(state)	\
1410	set_current_state(state)
1411
1412#define rwbase_restore_current_state()			\
1413	__set_current_state(TASK_RUNNING)
1414
1415#define rwbase_rtmutex_lock_state(rtm, state)		\
1416	__rt_mutex_lock(rtm, state)
1417
1418#define rwbase_rtmutex_slowlock_locked(rtm, state)	\
1419	__rt_mutex_slowlock_locked(rtm, NULL, state)
1420
1421#define rwbase_rtmutex_unlock(rtm)			\
1422	__rt_mutex_unlock(rtm)
1423
1424#define rwbase_rtmutex_trylock(rtm)			\
1425	__rt_mutex_trylock(rtm)
1426
1427#define rwbase_signal_pending_state(state, current)	\
1428	signal_pending_state(state, current)
1429
1430#define rwbase_pre_schedule()				\
1431	rt_mutex_pre_schedule()
1432
1433#define rwbase_schedule()				\
1434	rt_mutex_schedule()
1435
1436#define rwbase_post_schedule()				\
1437	rt_mutex_post_schedule()
1438
1439#include "rwbase_rt.c"
1440
1441void __init_rwsem(struct rw_semaphore *sem, const char *name,
1442		  struct lock_class_key *key)
1443{
1444	init_rwbase_rt(&(sem)->rwbase);
1445
1446#ifdef CONFIG_DEBUG_LOCK_ALLOC
1447	debug_check_no_locks_freed((void *)sem, sizeof(*sem));
1448	lockdep_init_map_wait(&sem->dep_map, name, key, 0, LD_WAIT_SLEEP);
1449#endif
1450}
1451EXPORT_SYMBOL(__init_rwsem);
1452
1453static inline void __down_read(struct rw_semaphore *sem)
1454{
1455	rwbase_read_lock(&sem->rwbase, TASK_UNINTERRUPTIBLE);
1456}
1457
1458static inline int __down_read_interruptible(struct rw_semaphore *sem)
1459{
1460	return rwbase_read_lock(&sem->rwbase, TASK_INTERRUPTIBLE);
1461}
1462
1463static inline int __down_read_killable(struct rw_semaphore *sem)
1464{
1465	return rwbase_read_lock(&sem->rwbase, TASK_KILLABLE);
1466}
1467
1468static inline int __down_read_trylock(struct rw_semaphore *sem)
1469{
1470	return rwbase_read_trylock(&sem->rwbase);
1471}
1472
1473static inline void __up_read(struct rw_semaphore *sem)
1474{
1475	rwbase_read_unlock(&sem->rwbase, TASK_NORMAL);
1476}
1477
1478static inline void __sched __down_write(struct rw_semaphore *sem)
1479{
1480	rwbase_write_lock(&sem->rwbase, TASK_UNINTERRUPTIBLE);
1481}
1482
1483static inline int __sched __down_write_killable(struct rw_semaphore *sem)
1484{
1485	return rwbase_write_lock(&sem->rwbase, TASK_KILLABLE);
1486}
1487
1488static inline int __down_write_trylock(struct rw_semaphore *sem)
1489{
1490	return rwbase_write_trylock(&sem->rwbase);
1491}
1492
1493static inline void __up_write(struct rw_semaphore *sem)
1494{
1495	rwbase_write_unlock(&sem->rwbase);
1496}
1497
1498static inline void __downgrade_write(struct rw_semaphore *sem)
1499{
1500	rwbase_write_downgrade(&sem->rwbase);
1501}
1502
1503/* Debug stubs for the common API */
1504#define DEBUG_RWSEMS_WARN_ON(c, sem)
1505
1506static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
1507					    struct task_struct *owner)
1508{
1509}
1510
1511static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
1512{
1513	int count = atomic_read(&sem->rwbase.readers);
1514
1515	return count < 0 && count != READER_BIAS;
1516}
1517
1518#endif /* CONFIG_PREEMPT_RT */
1519
1520/*
1521 * lock for reading
1522 */
1523void __sched down_read(struct rw_semaphore *sem)
1524{
1525	might_sleep();
1526	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1527
1528	LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
1529}
1530EXPORT_SYMBOL(down_read);
1531
1532int __sched down_read_interruptible(struct rw_semaphore *sem)
1533{
1534	might_sleep();
1535	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1536
1537	if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_interruptible)) {
1538		rwsem_release(&sem->dep_map, _RET_IP_);
1539		return -EINTR;
1540	}
1541
1542	return 0;
1543}
1544EXPORT_SYMBOL(down_read_interruptible);
1545
1546int __sched down_read_killable(struct rw_semaphore *sem)
1547{
1548	might_sleep();
1549	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1550
1551	if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
1552		rwsem_release(&sem->dep_map, _RET_IP_);
1553		return -EINTR;
1554	}
1555
1556	return 0;
1557}
1558EXPORT_SYMBOL(down_read_killable);
1559
1560/*
1561 * trylock for reading -- returns 1 if successful, 0 if contention
1562 */
1563int down_read_trylock(struct rw_semaphore *sem)
1564{
1565	int ret = __down_read_trylock(sem);
1566
1567	if (ret == 1)
1568		rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
1569	return ret;
1570}
1571EXPORT_SYMBOL(down_read_trylock);
1572
1573/*
1574 * lock for writing
1575 */
1576void __sched down_write(struct rw_semaphore *sem)
1577{
1578	might_sleep();
1579	rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1580	LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1581}
1582EXPORT_SYMBOL(down_write);
1583
1584/*
1585 * lock for writing
1586 */
1587int __sched down_write_killable(struct rw_semaphore *sem)
1588{
1589	might_sleep();
1590	rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1591
1592	if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
1593				  __down_write_killable)) {
1594		rwsem_release(&sem->dep_map, _RET_IP_);
1595		return -EINTR;
1596	}
1597
1598	return 0;
1599}
1600EXPORT_SYMBOL(down_write_killable);
1601
1602/*
1603 * trylock for writing -- returns 1 if successful, 0 if contention
1604 */
1605int down_write_trylock(struct rw_semaphore *sem)
1606{
1607	int ret = __down_write_trylock(sem);
1608
1609	if (ret == 1)
1610		rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
1611
1612	return ret;
1613}
1614EXPORT_SYMBOL(down_write_trylock);
1615
1616/*
1617 * release a read lock
1618 */
1619void up_read(struct rw_semaphore *sem)
1620{
1621	rwsem_release(&sem->dep_map, _RET_IP_);
1622	__up_read(sem);
1623}
1624EXPORT_SYMBOL(up_read);
1625
1626/*
1627 * release a write lock
1628 */
1629void up_write(struct rw_semaphore *sem)
1630{
1631	rwsem_release(&sem->dep_map, _RET_IP_);
1632	__up_write(sem);
1633}
1634EXPORT_SYMBOL(up_write);
1635
1636/*
1637 * downgrade write lock to read lock
1638 */
1639void downgrade_write(struct rw_semaphore *sem)
1640{
1641	lock_downgrade(&sem->dep_map, _RET_IP_);
1642	__downgrade_write(sem);
1643}
1644EXPORT_SYMBOL(downgrade_write);
1645
1646#ifdef CONFIG_DEBUG_LOCK_ALLOC
1647
1648void down_read_nested(struct rw_semaphore *sem, int subclass)
1649{
1650	might_sleep();
1651	rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
1652	LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
1653}
1654EXPORT_SYMBOL(down_read_nested);
1655
1656int down_read_killable_nested(struct rw_semaphore *sem, int subclass)
1657{
1658	might_sleep();
1659	rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
1660
1661	if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
1662		rwsem_release(&sem->dep_map, _RET_IP_);
1663		return -EINTR;
1664	}
1665
1666	return 0;
1667}
1668EXPORT_SYMBOL(down_read_killable_nested);
1669
1670void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
1671{
1672	might_sleep();
1673	rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
1674	LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1675}
1676EXPORT_SYMBOL(_down_write_nest_lock);
1677
1678void down_read_non_owner(struct rw_semaphore *sem)
1679{
1680	might_sleep();
1681	__down_read(sem);
1682	/*
1683	 * The owner value for a reader-owned lock is mostly for debugging
1684	 * purpose only and is not critical to the correct functioning of
1685	 * rwsem. So it is perfectly fine to set it in a preempt-enabled
1686	 * context here.
1687	 */
1688	__rwsem_set_reader_owned(sem, NULL);
1689}
1690EXPORT_SYMBOL(down_read_non_owner);
1691
1692void down_write_nested(struct rw_semaphore *sem, int subclass)
1693{
1694	might_sleep();
1695	rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
1696	LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1697}
1698EXPORT_SYMBOL(down_write_nested);
1699
1700int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
1701{
1702	might_sleep();
1703	rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
1704
1705	if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
1706				  __down_write_killable)) {
1707		rwsem_release(&sem->dep_map, _RET_IP_);
1708		return -EINTR;
1709	}
1710
1711	return 0;
1712}
1713EXPORT_SYMBOL(down_write_killable_nested);
1714
1715void up_read_non_owner(struct rw_semaphore *sem)
1716{
1717	DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1718	__up_read(sem);
1719}
1720EXPORT_SYMBOL(up_read_non_owner);
1721
1722#endif