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![Tejun Heo](/assets/img/avatar_default.png)
Currently, a percpu_ref which is initialized with PERPCU_REF_INIT_ATOMIC or switched to atomic mode via switch_to_atomic() automatically reverts to percpu mode on the first percpu_ref_reinit(). This makes the atomic mode difficult to use for cases where a percpu_ref is used as a persistent on/off switch which may be cycled multiple times. This patch makes such atomic state sticky so that it survives through kill/reinit cycles. After this patch, atomic state is cleared only by an explicit percpu_ref_switch_to_percpu() call. Signed-off-by: Tejun Heo <tj@kernel.org> Reviewed-by: Kent Overstreet <kmo@daterainc.com> Cc: Jens Axboe <axboe@kernel.dk> Cc: Christoph Hellwig <hch@infradead.org> Cc: Johannes Weiner <hannes@cmpxchg.org>
331 lines
12 KiB
C
331 lines
12 KiB
C
#define pr_fmt(fmt) "%s: " fmt "\n", __func__
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#include <linux/kernel.h>
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#include <linux/sched.h>
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#include <linux/wait.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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* percpu_count vars will be equal to what it would have been if all the gets
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* and 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
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* atomic_long_t can't hit 0 before we've added up all the percpu refs.
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*/
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#define PERCPU_COUNT_BIAS (1LU << (BITS_PER_LONG - 1))
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static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);
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static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
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{
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return (unsigned long __percpu *)
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(ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_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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* @flags: PERCPU_REF_INIT_* flags
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* @gfp: allocation mask to use
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*
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* Initializes @ref. If @flags is zero, @ref starts in percpu mode with a
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* refcount of 1; analagous to atomic_long_set(ref, 1). See the
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* definitions of PERCPU_REF_INIT_* flags for flag behaviors.
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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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unsigned int flags, gfp_t gfp)
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{
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size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
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__alignof__(unsigned long));
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unsigned long start_count = 0;
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ref->percpu_count_ptr = (unsigned long)
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__alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
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if (!ref->percpu_count_ptr)
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return -ENOMEM;
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ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
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if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD))
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ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
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else
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start_count += PERCPU_COUNT_BIAS;
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if (flags & PERCPU_REF_INIT_DEAD)
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ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
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else
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start_count++;
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atomic_long_set(&ref->count, start_count);
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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_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 long __percpu *percpu_count = percpu_count_ptr(ref);
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if (percpu_count) {
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free_percpu(percpu_count);
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ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_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_call_confirm_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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ref->confirm_switch(ref);
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ref->confirm_switch = NULL;
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wake_up_all(&percpu_ref_switch_waitq);
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/* drop ref from percpu_ref_switch_to_atomic() */
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percpu_ref_put(ref);
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}
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static void percpu_ref_switch_to_atomic_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 long __percpu *percpu_count = percpu_count_ptr(ref);
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unsigned long 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(percpu_count, cpu);
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pr_debug("global %ld percpu %ld",
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atomic_long_read(&ref->count), (long)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_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);
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WARN_ONCE(atomic_long_read(&ref->count) <= 0,
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"percpu ref (%pf) <= 0 (%ld) after switching to atomic",
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ref->release, atomic_long_read(&ref->count));
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/* @ref is viewed as dead on all CPUs, send out switch confirmation */
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percpu_ref_call_confirm_rcu(rcu);
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}
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static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
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{
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}
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static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
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percpu_ref_func_t *confirm_switch)
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{
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if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC)) {
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/* switching from percpu to atomic */
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ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
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/*
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* Non-NULL ->confirm_switch is used to indicate that
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* switching is in progress. Use noop one if unspecified.
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*/
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WARN_ON_ONCE(ref->confirm_switch);
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ref->confirm_switch =
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confirm_switch ?: percpu_ref_noop_confirm_switch;
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percpu_ref_get(ref); /* put after confirmation */
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call_rcu_sched(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
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} else if (confirm_switch) {
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/*
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* Somebody already set ATOMIC. Switching may still be in
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* progress. @confirm_switch must be invoked after the
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* switching is complete and a full sched RCU grace period
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* has passed. Wait synchronously for the previous
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* switching and schedule @confirm_switch invocation.
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*/
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wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
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ref->confirm_switch = confirm_switch;
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percpu_ref_get(ref); /* put after confirmation */
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call_rcu_sched(&ref->rcu, percpu_ref_call_confirm_rcu);
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}
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}
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/**
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* percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
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* @ref: percpu_ref to switch to atomic mode
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* @confirm_switch: optional confirmation callback
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*
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* There's no reason to use this function for the usual reference counting.
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* Use percpu_ref_kill[_and_confirm]().
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*
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* Schedule switching of @ref to atomic mode. All its percpu counts will
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* be collected to the main atomic counter. On completion, when all CPUs
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* are guaraneed to be in atomic mode, @confirm_switch, which may not
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* block, is invoked. This function may be invoked concurrently with all
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* the get/put operations and can safely be mixed with kill and reinit
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* operations. Note that @ref will stay in atomic mode across kill/reinit
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* cycles until percpu_ref_switch_to_percpu() is called.
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*
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* This function normally doesn't block and can be called from any context
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* but it may block if @confirm_kill is specified and @ref is already in
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* the process of switching to atomic mode. In such cases, @confirm_switch
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* will be invoked after the switching is complete.
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*
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* Due to the way percpu_ref is implemented, @confirm_switch will be called
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* after at least one full sched RCU grace period has passed but this is an
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* implementation detail and must not be depended upon.
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*/
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void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
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percpu_ref_func_t *confirm_switch)
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{
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ref->force_atomic = true;
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__percpu_ref_switch_to_atomic(ref, confirm_switch);
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}
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static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
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{
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unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
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int cpu;
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BUG_ON(!percpu_count);
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if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
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return;
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wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
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atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
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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 __ref_is_percpu() and guarantees
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* that the zeroing is visible to all percpu accesses which can see
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* the following __PERCPU_REF_ATOMIC clearing.
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*/
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for_each_possible_cpu(cpu)
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*per_cpu_ptr(percpu_count, cpu) = 0;
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smp_store_release(&ref->percpu_count_ptr,
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ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
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}
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/**
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* percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
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* @ref: percpu_ref to switch to percpu mode
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*
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* There's no reason to use this function for the usual reference counting.
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* To re-use an expired ref, use percpu_ref_reinit().
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*
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* Switch @ref to percpu mode. This function may be invoked concurrently
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* with all the get/put operations and can safely be mixed with kill and
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* reinit operations. This function reverses the sticky atomic state set
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* by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic(). If @ref is
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* dying or dead, the actual switching takes place on the following
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* percpu_ref_reinit().
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*
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* This function normally doesn't block and can be called from any context
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* but it may block if @ref is in the process of switching to atomic mode
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* by percpu_ref_switch_atomic().
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*/
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void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
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{
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ref->force_atomic = false;
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/* a dying or dead ref can't be switched to percpu mode w/o reinit */
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if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD))
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__percpu_ref_switch_to_percpu(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 at which point all
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* further invocations of percpu_ref_tryget_live() will fail. See
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* percpu_ref_tryget_live() for details.
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*
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* This function normally doesn't block and can be called from any context
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* but it may block if @confirm_kill is specified and @ref is in the
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* process of switching to atomic mode by percpu_ref_switch_atomic().
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*
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* Due to the way percpu_ref is implemented, @confirm_switch will be called
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* after at least one full sched RCU grace period has passed but this is an
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* implementation detail and must not be depended upon.
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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->percpu_count_ptr & __PERCPU_REF_DEAD,
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"%s called more than once on %pf!", __func__, ref->release);
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ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
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__percpu_ref_switch_to_atomic(ref, confirm_kill);
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percpu_ref_put(ref);
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}
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EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
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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() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been
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* initialized successfully 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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WARN_ON_ONCE(!percpu_ref_is_zero(ref));
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ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
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percpu_ref_get(ref);
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if (!ref->force_atomic)
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__percpu_ref_switch_to_percpu(ref);
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}
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EXPORT_SYMBOL_GPL(percpu_ref_reinit);
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