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1/*
2 * Queued spinlock
3 *
4 * This program is free software; you can redistribute it and/or modify
5 * it under the terms of the GNU General Public License as published by
6 * the Free Software Foundation; either version 2 of the License, or
7 * (at your option) any later version.
8 *
9 * This program is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 * GNU General Public License for more details.
13 *
14 * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
15 * (C) Copyright 2013-2014 Red Hat, Inc.
16 * (C) Copyright 2015 Intel Corp.
17 * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
18 *
19 * Authors: Waiman Long <waiman.long@hpe.com>
20 * Peter Zijlstra <peterz@infradead.org>
21 */
22
23#ifndef _GEN_PV_LOCK_SLOWPATH
24
25#include <linux/smp.h>
26#include <linux/bug.h>
27#include <linux/cpumask.h>
28#include <linux/percpu.h>
29#include <linux/hardirq.h>
30#include <linux/mutex.h>
31#include <linux/prefetch.h>
32#include <asm/byteorder.h>
33#include <asm/qspinlock.h>
34
35/*
36 * The basic principle of a queue-based spinlock can best be understood
37 * by studying a classic queue-based spinlock implementation called the
38 * MCS lock. The paper below provides a good description for this kind
39 * of lock.
40 *
41 * http://www.cise.ufl.edu/tr/DOC/REP-1992-71.pdf
42 *
43 * This queued spinlock implementation is based on the MCS lock, however to make
44 * it fit the 4 bytes we assume spinlock_t to be, and preserve its existing
45 * API, we must modify it somehow.
46 *
47 * In particular; where the traditional MCS lock consists of a tail pointer
48 * (8 bytes) and needs the next pointer (another 8 bytes) of its own node to
49 * unlock the next pending (next->locked), we compress both these: {tail,
50 * next->locked} into a single u32 value.
51 *
52 * Since a spinlock disables recursion of its own context and there is a limit
53 * to the contexts that can nest; namely: task, softirq, hardirq, nmi. As there
54 * are at most 4 nesting levels, it can be encoded by a 2-bit number. Now
55 * we can encode the tail by combining the 2-bit nesting level with the cpu
56 * number. With one byte for the lock value and 3 bytes for the tail, only a
57 * 32-bit word is now needed. Even though we only need 1 bit for the lock,
58 * we extend it to a full byte to achieve better performance for architectures
59 * that support atomic byte write.
60 *
61 * We also change the first spinner to spin on the lock bit instead of its
62 * node; whereby avoiding the need to carry a node from lock to unlock, and
63 * preserving existing lock API. This also makes the unlock code simpler and
64 * faster.
65 *
66 * N.B. The current implementation only supports architectures that allow
67 * atomic operations on smaller 8-bit and 16-bit data types.
68 *
69 */
70
71#include "mcs_spinlock.h"
72
73#ifdef CONFIG_PARAVIRT_SPINLOCKS
74#define MAX_NODES 8
75#else
76#define MAX_NODES 4
77#endif
78
79/*
80 * Per-CPU queue node structures; we can never have more than 4 nested
81 * contexts: task, softirq, hardirq, nmi.
82 *
83 * Exactly fits one 64-byte cacheline on a 64-bit architecture.
84 *
85 * PV doubles the storage and uses the second cacheline for PV state.
86 */
87static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[MAX_NODES]);
88
89/*
90 * We must be able to distinguish between no-tail and the tail at 0:0,
91 * therefore increment the cpu number by one.
92 */
93
94static inline __pure u32 encode_tail(int cpu, int idx)
95{
96 u32 tail;
97
98#ifdef CONFIG_DEBUG_SPINLOCK
99 BUG_ON(idx > 3);
100#endif
101 tail = (cpu + 1) << _Q_TAIL_CPU_OFFSET;
102 tail |= idx << _Q_TAIL_IDX_OFFSET; /* assume < 4 */
103
104 return tail;
105}
106
107static inline __pure struct mcs_spinlock *decode_tail(u32 tail)
108{
109 int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1;
110 int idx = (tail & _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET;
111
112 return per_cpu_ptr(&mcs_nodes[idx], cpu);
113}
114
115#define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)
116
117/*
118 * By using the whole 2nd least significant byte for the pending bit, we
119 * can allow better optimization of the lock acquisition for the pending
120 * bit holder.
121 *
122 * This internal structure is also used by the set_locked function which
123 * is not restricted to _Q_PENDING_BITS == 8.
124 */
125struct __qspinlock {
126 union {
127 atomic_t val;
128#ifdef __LITTLE_ENDIAN
129 struct {
130 u8 locked;
131 u8 pending;
132 };
133 struct {
134 u16 locked_pending;
135 u16 tail;
136 };
137#else
138 struct {
139 u16 tail;
140 u16 locked_pending;
141 };
142 struct {
143 u8 reserved[2];
144 u8 pending;
145 u8 locked;
146 };
147#endif
148 };
149};
150
151#if _Q_PENDING_BITS == 8
152/**
153 * clear_pending_set_locked - take ownership and clear the pending bit.
154 * @lock: Pointer to queued spinlock structure
155 *
156 * *,1,0 -> *,0,1
157 *
158 * Lock stealing is not allowed if this function is used.
159 */
160static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
161{
162 struct __qspinlock *l = (void *)lock;
163
164 WRITE_ONCE(l->locked_pending, _Q_LOCKED_VAL);
165}
166
167/*
168 * xchg_tail - Put in the new queue tail code word & retrieve previous one
169 * @lock : Pointer to queued spinlock structure
170 * @tail : The new queue tail code word
171 * Return: The previous queue tail code word
172 *
173 * xchg(lock, tail), which heads an address dependency
174 *
175 * p,*,* -> n,*,* ; prev = xchg(lock, node)
176 */
177static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
178{
179 struct __qspinlock *l = (void *)lock;
180
181 /*
182 * Use release semantics to make sure that the MCS node is properly
183 * initialized before changing the tail code.
184 */
185 return (u32)xchg_release(&l->tail,
186 tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
187}
188
189#else /* _Q_PENDING_BITS == 8 */
190
191/**
192 * clear_pending_set_locked - take ownership and clear the pending bit.
193 * @lock: Pointer to queued spinlock structure
194 *
195 * *,1,0 -> *,0,1
196 */
197static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
198{
199 atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val);
200}
201
202/**
203 * xchg_tail - Put in the new queue tail code word & retrieve previous one
204 * @lock : Pointer to queued spinlock structure
205 * @tail : The new queue tail code word
206 * Return: The previous queue tail code word
207 *
208 * xchg(lock, tail)
209 *
210 * p,*,* -> n,*,* ; prev = xchg(lock, node)
211 */
212static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
213{
214 u32 old, new, val = atomic_read(&lock->val);
215
216 for (;;) {
217 new = (val & _Q_LOCKED_PENDING_MASK) | tail;
218 /*
219 * Use release semantics to make sure that the MCS node is
220 * properly initialized before changing the tail code.
221 */
222 old = atomic_cmpxchg_release(&lock->val, val, new);
223 if (old == val)
224 break;
225
226 val = old;
227 }
228 return old;
229}
230#endif /* _Q_PENDING_BITS == 8 */
231
232/**
233 * set_locked - Set the lock bit and own the lock
234 * @lock: Pointer to queued spinlock structure
235 *
236 * *,*,0 -> *,0,1
237 */
238static __always_inline void set_locked(struct qspinlock *lock)
239{
240 struct __qspinlock *l = (void *)lock;
241
242 WRITE_ONCE(l->locked, _Q_LOCKED_VAL);
243}
244
245
246/*
247 * Generate the native code for queued_spin_unlock_slowpath(); provide NOPs for
248 * all the PV callbacks.
249 */
250
251static __always_inline void __pv_init_node(struct mcs_spinlock *node) { }
252static __always_inline void __pv_wait_node(struct mcs_spinlock *node,
253 struct mcs_spinlock *prev) { }
254static __always_inline void __pv_kick_node(struct qspinlock *lock,
255 struct mcs_spinlock *node) { }
256static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock,
257 struct mcs_spinlock *node)
258 { return 0; }
259
260#define pv_enabled() false
261
262#define pv_init_node __pv_init_node
263#define pv_wait_node __pv_wait_node
264#define pv_kick_node __pv_kick_node
265#define pv_wait_head_or_lock __pv_wait_head_or_lock
266
267#ifdef CONFIG_PARAVIRT_SPINLOCKS
268#define queued_spin_lock_slowpath native_queued_spin_lock_slowpath
269#endif
270
271#endif /* _GEN_PV_LOCK_SLOWPATH */
272
273/**
274 * queued_spin_lock_slowpath - acquire the queued spinlock
275 * @lock: Pointer to queued spinlock structure
276 * @val: Current value of the queued spinlock 32-bit word
277 *
278 * (queue tail, pending bit, lock value)
279 *
280 * fast : slow : unlock
281 * : :
282 * uncontended (0,0,0) -:--> (0,0,1) ------------------------------:--> (*,*,0)
283 * : | ^--------.------. / :
284 * : v \ \ | :
285 * pending : (0,1,1) +--> (0,1,0) \ | :
286 * : | ^--' | | :
287 * : v | | :
288 * uncontended : (n,x,y) +--> (n,0,0) --' | :
289 * queue : | ^--' | :
290 * : v | :
291 * contended : (*,x,y) +--> (*,0,0) ---> (*,0,1) -' :
292 * queue : ^--' :
293 */
294void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
295{
296 struct mcs_spinlock *prev, *next, *node;
297 u32 new, old, tail;
298 int idx;
299
300 BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
301
302 if (pv_enabled())
303 goto queue;
304
305 if (virt_spin_lock(lock))
306 return;
307
308 /*
309 * wait for in-progress pending->locked hand-overs
310 *
311 * 0,1,0 -> 0,0,1
312 */
313 if (val == _Q_PENDING_VAL) {
314 while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL)
315 cpu_relax();
316 }
317
318 /*
319 * trylock || pending
320 *
321 * 0,0,0 -> 0,0,1 ; trylock
322 * 0,0,1 -> 0,1,1 ; pending
323 */
324 for (;;) {
325 /*
326 * If we observe any contention; queue.
327 */
328 if (val & ~_Q_LOCKED_MASK)
329 goto queue;
330
331 new = _Q_LOCKED_VAL;
332 if (val == new)
333 new |= _Q_PENDING_VAL;
334
335 /*
336 * Acquire semantic is required here as the function may
337 * return immediately if the lock was free.
338 */
339 old = atomic_cmpxchg_acquire(&lock->val, val, new);
340 if (old == val)
341 break;
342
343 val = old;
344 }
345
346 /*
347 * we won the trylock
348 */
349 if (new == _Q_LOCKED_VAL)
350 return;
351
352 /*
353 * we're pending, wait for the owner to go away.
354 *
355 * *,1,1 -> *,1,0
356 *
357 * this wait loop must be a load-acquire such that we match the
358 * store-release that clears the locked bit and create lock
359 * sequentiality; this is because not all clear_pending_set_locked()
360 * implementations imply full barriers.
361 */
362 smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK));
363
364 /*
365 * take ownership and clear the pending bit.
366 *
367 * *,1,0 -> *,0,1
368 */
369 clear_pending_set_locked(lock);
370 return;
371
372 /*
373 * End of pending bit optimistic spinning and beginning of MCS
374 * queuing.
375 */
376queue:
377 node = this_cpu_ptr(&mcs_nodes[0]);
378 idx = node->count++;
379 tail = encode_tail(smp_processor_id(), idx);
380
381 node += idx;
382
383 /*
384 * Ensure that we increment the head node->count before initialising
385 * the actual node. If the compiler is kind enough to reorder these
386 * stores, then an IRQ could overwrite our assignments.
387 */
388 barrier();
389
390 node->locked = 0;
391 node->next = NULL;
392 pv_init_node(node);
393
394 /*
395 * We touched a (possibly) cold cacheline in the per-cpu queue node;
396 * attempt the trylock once more in the hope someone let go while we
397 * weren't watching.
398 */
399 if (queued_spin_trylock(lock))
400 goto release;
401
402 /*
403 * We have already touched the queueing cacheline; don't bother with
404 * pending stuff.
405 *
406 * p,*,* -> n,*,*
407 *
408 * RELEASE, such that the stores to @node must be complete.
409 */
410 old = xchg_tail(lock, tail);
411 next = NULL;
412
413 /*
414 * if there was a previous node; link it and wait until reaching the
415 * head of the waitqueue.
416 */
417 if (old & _Q_TAIL_MASK) {
418 prev = decode_tail(old);
419
420 /*
421 * We must ensure that the stores to @node are observed before
422 * the write to prev->next. The address dependency from
423 * xchg_tail is not sufficient to ensure this because the read
424 * component of xchg_tail is unordered with respect to the
425 * initialisation of @node.
426 */
427 smp_store_release(&prev->next, node);
428
429 pv_wait_node(node, prev);
430 arch_mcs_spin_lock_contended(&node->locked);
431
432 /*
433 * While waiting for the MCS lock, the next pointer may have
434 * been set by another lock waiter. We optimistically load
435 * the next pointer & prefetch the cacheline for writing
436 * to reduce latency in the upcoming MCS unlock operation.
437 */
438 next = READ_ONCE(node->next);
439 if (next)
440 prefetchw(next);
441 }
442
443 /*
444 * we're at the head of the waitqueue, wait for the owner & pending to
445 * go away.
446 *
447 * *,x,y -> *,0,0
448 *
449 * this wait loop must use a load-acquire such that we match the
450 * store-release that clears the locked bit and create lock
451 * sequentiality; this is because the set_locked() function below
452 * does not imply a full barrier.
453 *
454 * The PV pv_wait_head_or_lock function, if active, will acquire
455 * the lock and return a non-zero value. So we have to skip the
456 * smp_cond_load_acquire() call. As the next PV queue head hasn't been
457 * designated yet, there is no way for the locked value to become
458 * _Q_SLOW_VAL. So both the set_locked() and the
459 * atomic_cmpxchg_relaxed() calls will be safe.
460 *
461 * If PV isn't active, 0 will be returned instead.
462 *
463 */
464 if ((val = pv_wait_head_or_lock(lock, node)))
465 goto locked;
466
467 val = smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_PENDING_MASK));
468
469locked:
470 /*
471 * claim the lock:
472 *
473 * n,0,0 -> 0,0,1 : lock, uncontended
474 * *,0,0 -> *,0,1 : lock, contended
475 *
476 * If the queue head is the only one in the queue (lock value == tail),
477 * clear the tail code and grab the lock. Otherwise, we only need
478 * to grab the lock.
479 */
480 for (;;) {
481 /* In the PV case we might already have _Q_LOCKED_VAL set */
482 if ((val & _Q_TAIL_MASK) != tail) {
483 set_locked(lock);
484 break;
485 }
486 /*
487 * The smp_cond_load_acquire() call above has provided the
488 * necessary acquire semantics required for locking. At most
489 * two iterations of this loop may be ran.
490 */
491 old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
492 if (old == val)
493 goto release; /* No contention */
494
495 val = old;
496 }
497
498 /*
499 * contended path; wait for next if not observed yet, release.
500 */
501 if (!next) {
502 while (!(next = READ_ONCE(node->next)))
503 cpu_relax();
504 }
505
506 arch_mcs_spin_unlock_contended(&next->locked);
507 pv_kick_node(lock, next);
508
509release:
510 /*
511 * release the node
512 */
513 __this_cpu_dec(mcs_nodes[0].count);
514}
515EXPORT_SYMBOL(queued_spin_lock_slowpath);
516
517/*
518 * Generate the paravirt code for queued_spin_unlock_slowpath().
519 */
520#if !defined(_GEN_PV_LOCK_SLOWPATH) && defined(CONFIG_PARAVIRT_SPINLOCKS)
521#define _GEN_PV_LOCK_SLOWPATH
522
523#undef pv_enabled
524#define pv_enabled() true
525
526#undef pv_init_node
527#undef pv_wait_node
528#undef pv_kick_node
529#undef pv_wait_head_or_lock
530
531#undef queued_spin_lock_slowpath
532#define queued_spin_lock_slowpath __pv_queued_spin_lock_slowpath
533
534#include "qspinlock_paravirt.h"
535#include "qspinlock.c"
536
537#endif