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![Matias Bjorling](/assets/img/avatar_default.png)
Export the interface to be used within modules. Signed-off-by: Matias Bjorling <m@bjorling.me> Acked-by: Tejun Heo <tj@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
161 lines
5.7 KiB
C
161 lines
5.7 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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/**
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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 = alloc_percpu(unsigned);
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if (!ref->pcpu_count)
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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_cancel_init - cancel percpu_ref_init()
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* @ref: percpu_ref to cancel init for
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*
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* Once a percpu_ref is initialized, its destruction is initiated by
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* percpu_ref_kill() and completes asynchronously, which can be painful to
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* do when destroying a half-constructed object in init failure path.
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*
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* This function destroys @ref without invoking @ref->release and the
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* memory area containing it can be freed immediately on return. To
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* prevent accidental misuse, it's required that @ref has finished
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* percpu_ref_init(), whether successful or not, but never used.
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*
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* The weird name and usage restriction are to prevent people from using
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* this function by mistake for normal shutdown instead of
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* percpu_ref_kill().
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*/
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void percpu_ref_cancel_init(struct percpu_ref *ref)
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{
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unsigned __percpu *pcpu_count = ref->pcpu_count;
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int cpu;
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WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS);
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if (pcpu_count) {
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for_each_possible_cpu(cpu)
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WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu));
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free_percpu(ref->pcpu_count);
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}
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}
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EXPORT_SYMBOL_GPL(percpu_ref_cancel_init);
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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 = ref->pcpu_count;
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unsigned count = 0;
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int cpu;
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/* Mask out PCPU_REF_DEAD */
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pcpu_count = (unsigned __percpu *)
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(((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
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for_each_possible_cpu(cpu)
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count += *per_cpu_ptr(pcpu_count, cpu);
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free_percpu(pcpu_count);
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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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/* @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_STATUS(ref->pcpu_count) == PCPU_REF_DEAD,
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"percpu_ref_kill() called more than once!\n");
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ref->pcpu_count = (unsigned __percpu *)
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(((unsigned long) ref->pcpu_count)|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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