1#define pr_fmt(fmt) "%s: " fmt "\n", __func__
  2
  3#include <linux/kernel.h>
  4#include <linux/percpu-refcount.h>
  5
  6/*
  7 * Initially, a percpu refcount is just a set of percpu counters. Initially, we
  8 * don't try to detect the ref hitting 0 - which means that get/put can just
  9 * increment or decrement the local counter. Note that the counter on a
 10 * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
 11 * percpu counters will all sum to the correct value
 12 *
 13 * (More precisely: because moduler arithmatic is commutative the sum of all the
 14 * pcpu_count vars will be equal to what it would have been if all the gets and
 15 * puts were done to a single integer, even if some of the percpu integers
 16 * overflow or underflow).
 17 *
 18 * The real trick to implementing percpu refcounts is shutdown. We can't detect
 19 * the ref hitting 0 on every put - this would require global synchronization
 20 * and defeat the whole purpose of using percpu refs.
 21 *
 22 * What we do is require the user to keep track of the initial refcount; we know
 23 * the ref can't hit 0 before the user drops the initial ref, so as long as we
 24 * convert to non percpu mode before the initial ref is dropped everything
 25 * works.
 26 *
 27 * Converting to non percpu mode is done with some RCUish stuff in
 28 * percpu_ref_kill. Additionally, we need a bias value so that the atomic_t
 29 * can't hit 0 before we've added up all the percpu refs.
 30 */
 31
 32#define PCPU_COUNT_BIAS		(1U << 31)
 33
 34/**
 35 * percpu_ref_init - initialize a percpu refcount
 36 * @ref: percpu_ref to initialize
 37 * @release: function which will be called when refcount hits 0
 38 *
 39 * Initializes the refcount in single atomic counter mode with a refcount of 1;
 40 * analagous to atomic_set(ref, 1).
 41 *
 42 * Note that @release must not sleep - it may potentially be called from RCU
 43 * callback context by percpu_ref_kill().
 44 */
 45int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release)
 46{
 47	atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
 48
 49	ref->pcpu_count = alloc_percpu(unsigned);
 50	if (!ref->pcpu_count)
 51		return -ENOMEM;
 52
 53	ref->release = release;
 54	return 0;
 55}
 56EXPORT_SYMBOL_GPL(percpu_ref_init);
 57
 58/**
 59 * percpu_ref_cancel_init - cancel percpu_ref_init()
 60 * @ref: percpu_ref to cancel init for
 61 *
 62 * Once a percpu_ref is initialized, its destruction is initiated by
 63 * percpu_ref_kill() and completes asynchronously, which can be painful to
 64 * do when destroying a half-constructed object in init failure path.
 65 *
 66 * This function destroys @ref without invoking @ref->release and the
 67 * memory area containing it can be freed immediately on return.  To
 68 * prevent accidental misuse, it's required that @ref has finished
 69 * percpu_ref_init(), whether successful or not, but never used.
 70 *
 71 * The weird name and usage restriction are to prevent people from using
 72 * this function by mistake for normal shutdown instead of
 73 * percpu_ref_kill().
 74 */
 75void percpu_ref_cancel_init(struct percpu_ref *ref)
 76{
 77	unsigned __percpu *pcpu_count = ref->pcpu_count;
 78	int cpu;
 79
 80	WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS);
 81
 82	if (pcpu_count) {
 83		for_each_possible_cpu(cpu)
 84			WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu));
 85		free_percpu(ref->pcpu_count);
 86	}
 87}
 88EXPORT_SYMBOL_GPL(percpu_ref_cancel_init);
 89
 90static void percpu_ref_kill_rcu(struct rcu_head *rcu)
 91{
 92	struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
 93	unsigned __percpu *pcpu_count = ref->pcpu_count;
 94	unsigned count = 0;
 95	int cpu;
 96
 97	/* Mask out PCPU_REF_DEAD */
 98	pcpu_count = (unsigned __percpu *)
 99		(((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
100
101	for_each_possible_cpu(cpu)
102		count += *per_cpu_ptr(pcpu_count, cpu);
103
104	free_percpu(pcpu_count);
105
106	pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count);
107
108	/*
109	 * It's crucial that we sum the percpu counters _before_ adding the sum
110	 * to &ref->count; since gets could be happening on one cpu while puts
111	 * happen on another, adding a single cpu's count could cause
112	 * @ref->count to hit 0 before we've got a consistent value - but the
113	 * sum of all the counts will be consistent and correct.
114	 *
115	 * Subtracting the bias value then has to happen _after_ adding count to
116	 * &ref->count; we need the bias value to prevent &ref->count from
117	 * reaching 0 before we add the percpu counts. But doing it at the same
118	 * time is equivalent and saves us atomic operations:
119	 */
120
121	atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
122
123	WARN_ONCE(atomic_read(&ref->count) <= 0, "percpu ref <= 0 (%i)",
124		  atomic_read(&ref->count));
125
126	/* @ref is viewed as dead on all CPUs, send out kill confirmation */
127	if (ref->confirm_kill)
128		ref->confirm_kill(ref);
129
130	/*
131	 * Now we're in single atomic_t mode with a consistent refcount, so it's
132	 * safe to drop our initial ref:
133	 */
134	percpu_ref_put(ref);
135}
136
137/**
138 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
139 * @ref: percpu_ref to kill
140 * @confirm_kill: optional confirmation callback
141 *
142 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
143 * @confirm_kill is not NULL.  @confirm_kill, which may not block, will be
144 * called after @ref is seen as dead from all CPUs - all further
145 * invocations of percpu_ref_tryget() will fail.  See percpu_ref_tryget()
146 * for more details.
147 *
148 * Due to the way percpu_ref is implemented, @confirm_kill will be called
149 * after at least one full RCU grace period has passed but this is an
150 * implementation detail and callers must not depend on it.
151 */
152void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
153				 percpu_ref_func_t *confirm_kill)
154{
155	WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD,
156		  "percpu_ref_kill() called more than once!\n");
157
158	ref->pcpu_count = (unsigned __percpu *)
159		(((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);
160	ref->confirm_kill = confirm_kill;
161
162	call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);
163}
164EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);