203 lines
6.8 KiB
C
203 lines
6.8 KiB
C
#define pr_fmt(fmt) "%s: " fmt "\n", __func__
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#include <linux/kernel.h>
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#include <linux/percpu-refcount.h>
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/*
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* Initially, a percpu refcount is just a set of percpu counters. Initially, we
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* don't try to detect the ref hitting 0 - which means that get/put can just
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* increment or decrement the local counter. Note that the counter on a
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* particular cpu can (and will) wrap - this is fine, when we go to shutdown the
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* percpu counters will all sum to the correct value
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*
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* (More precisely: because moduler arithmatic is commutative the sum of all the
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* pcpu_count vars will be equal to what it would have been if all the gets and
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* puts were done to a single integer, even if some of the percpu integers
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* overflow or underflow).
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*
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* The real trick to implementing percpu refcounts is shutdown. We can't detect
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* the ref hitting 0 on every put - this would require global synchronization
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* and defeat the whole purpose of using percpu refs.
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*
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* What we do is require the user to keep track of the initial refcount; we know
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* the ref can't hit 0 before the user drops the initial ref, so as long as we
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* convert to non percpu mode before the initial ref is dropped everything
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* works.
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*
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* Converting to non percpu mode is done with some RCUish stuff in
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* percpu_ref_kill. Additionally, we need a bias value so that the atomic_t
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* can't hit 0 before we've added up all the percpu refs.
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*/
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#define PCPU_COUNT_BIAS (1U << 31)
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static unsigned __percpu *pcpu_count_ptr(struct percpu_ref *ref)
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{
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return (unsigned __percpu *)(ref->pcpu_count_ptr & ~PCPU_REF_DEAD);
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}
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/**
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* percpu_ref_init - initialize a percpu refcount
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* @ref: percpu_ref to initialize
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* @release: function which will be called when refcount hits 0
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*
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* Initializes the refcount in single atomic counter mode with a refcount of 1;
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* analagous to atomic_set(ref, 1).
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*
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* Note that @release must not sleep - it may potentially be called from RCU
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* callback context by percpu_ref_kill().
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*/
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int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release)
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{
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atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
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ref->pcpu_count_ptr = (unsigned long)alloc_percpu(unsigned);
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if (!ref->pcpu_count_ptr)
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return -ENOMEM;
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ref->release = release;
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return 0;
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}
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EXPORT_SYMBOL_GPL(percpu_ref_init);
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/**
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* percpu_ref_reinit - re-initialize a percpu refcount
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* @ref: perpcu_ref to re-initialize
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*
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* Re-initialize @ref so that it's in the same state as when it finished
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* percpu_ref_init(). @ref must have been initialized successfully, killed
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* and reached 0 but not exited.
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*
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* Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
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* this function is in progress.
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*/
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void percpu_ref_reinit(struct percpu_ref *ref)
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{
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unsigned __percpu *pcpu_count = pcpu_count_ptr(ref);
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int cpu;
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BUG_ON(!pcpu_count);
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WARN_ON(!percpu_ref_is_zero(ref));
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atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
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/*
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* Restore per-cpu operation. smp_store_release() is paired with
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* smp_read_barrier_depends() in __pcpu_ref_alive() and guarantees
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* that the zeroing is visible to all percpu accesses which can see
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* the following PCPU_REF_DEAD clearing.
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*/
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for_each_possible_cpu(cpu)
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*per_cpu_ptr(pcpu_count, cpu) = 0;
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smp_store_release(&ref->pcpu_count_ptr,
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ref->pcpu_count_ptr & ~PCPU_REF_DEAD);
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}
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EXPORT_SYMBOL_GPL(percpu_ref_reinit);
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/**
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* percpu_ref_exit - undo percpu_ref_init()
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* @ref: percpu_ref to exit
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*
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* This function exits @ref. The caller is responsible for ensuring that
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* @ref is no longer in active use. The usual places to invoke this
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* function from are the @ref->release() callback or in init failure path
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* where percpu_ref_init() succeeded but other parts of the initialization
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* of the embedding object failed.
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*/
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void percpu_ref_exit(struct percpu_ref *ref)
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{
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unsigned __percpu *pcpu_count = pcpu_count_ptr(ref);
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if (pcpu_count) {
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free_percpu(pcpu_count);
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ref->pcpu_count_ptr = PCPU_REF_DEAD;
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}
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}
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EXPORT_SYMBOL_GPL(percpu_ref_exit);
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static void percpu_ref_kill_rcu(struct rcu_head *rcu)
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{
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struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
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unsigned __percpu *pcpu_count = pcpu_count_ptr(ref);
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unsigned count = 0;
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int cpu;
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for_each_possible_cpu(cpu)
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count += *per_cpu_ptr(pcpu_count, cpu);
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pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count);
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/*
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* It's crucial that we sum the percpu counters _before_ adding the sum
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* to &ref->count; since gets could be happening on one cpu while puts
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* happen on another, adding a single cpu's count could cause
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* @ref->count to hit 0 before we've got a consistent value - but the
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* sum of all the counts will be consistent and correct.
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*
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* Subtracting the bias value then has to happen _after_ adding count to
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* &ref->count; we need the bias value to prevent &ref->count from
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* reaching 0 before we add the percpu counts. But doing it at the same
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* time is equivalent and saves us atomic operations:
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*/
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atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
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WARN_ONCE(atomic_read(&ref->count) <= 0, "percpu ref <= 0 (%i)",
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atomic_read(&ref->count));
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/* @ref is viewed as dead on all CPUs, send out kill confirmation */
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if (ref->confirm_kill)
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ref->confirm_kill(ref);
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/*
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* Now we're in single atomic_t mode with a consistent refcount, so it's
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* safe to drop our initial ref:
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*/
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percpu_ref_put(ref);
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}
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/**
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* percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
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* @ref: percpu_ref to kill
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* @confirm_kill: optional confirmation callback
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*
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* Equivalent to percpu_ref_kill() but also schedules kill confirmation if
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* @confirm_kill is not NULL. @confirm_kill, which may not block, will be
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* called after @ref is seen as dead from all CPUs - all further
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* invocations of percpu_ref_tryget() will fail. See percpu_ref_tryget()
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* for more details.
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*
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* Due to the way percpu_ref is implemented, @confirm_kill will be called
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* after at least one full RCU grace period has passed but this is an
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* implementation detail and callers must not depend on it.
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*/
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void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
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percpu_ref_func_t *confirm_kill)
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{
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WARN_ONCE(ref->pcpu_count_ptr & PCPU_REF_DEAD,
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"percpu_ref_kill() called more than once!\n");
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ref->pcpu_count_ptr |= PCPU_REF_DEAD;
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ref->confirm_kill = confirm_kill;
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call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);
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}
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EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
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/*
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* XXX: Temporary kludge to work around SCSI blk-mq stall. Used only by
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* block/blk-mq.c::blk_mq_freeze_queue(). Will be removed during v3.18
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* devel cycle. Do not use anywhere else.
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*/
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void __percpu_ref_kill_expedited(struct percpu_ref *ref)
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{
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WARN_ONCE(ref->pcpu_count_ptr & PCPU_REF_DEAD,
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"percpu_ref_kill() called more than once on %pf!",
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ref->release);
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ref->pcpu_count_ptr |= PCPU_REF_DEAD;
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synchronize_sched_expedited();
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percpu_ref_kill_rcu(&ref->rcu);
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}
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