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![NeilBrown](/assets/img/avatar_default.png)
percpu_ref_switch_to_atomic_sync() schedules the switch to atomic mode, then waits for it to complete. Also export percpu_ref_switch_to_* so they can be used from modules. This will be used in md/raid to count the number of pending write requests to an array. We occasionally need to check if the count is zero, but most often we don't care. We always want updates to the counter to be fast, as in some cases we count every 4K page. Signed-off-by: NeilBrown <neilb@suse.com> Acked-by: Tejun Heo <tj@kernel.org> Signed-off-by: Shaohua Li <shli@fb.com>
369 lines
12 KiB
C
369 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 modular arithmetic 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 DEFINE_SPINLOCK(percpu_ref_switch_lock);
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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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ref->confirm_switch = NULL;
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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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/* non-NULL confirm_switch indicates switching in progress */
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WARN_ON_ONCE(ref->confirm_switch);
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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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if (confirm_switch)
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confirm_switch(ref);
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return;
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}
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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 switching is
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* in progress. Use noop one if unspecified.
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*/
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ref->confirm_switch = 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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}
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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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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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static void __percpu_ref_switch_mode(struct percpu_ref *ref,
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percpu_ref_func_t *confirm_switch)
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{
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lockdep_assert_held(&percpu_ref_switch_lock);
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/*
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* If the previous ATOMIC switching hasn't finished yet, wait for
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* its completion. If the caller ensures that ATOMIC switching
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* isn't in progress, this function can be called from any context.
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*/
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wait_event_lock_irq(percpu_ref_switch_waitq, !ref->confirm_switch,
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percpu_ref_switch_lock);
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if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))
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__percpu_ref_switch_to_atomic(ref, confirm_switch);
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else
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__percpu_ref_switch_to_percpu(ref);
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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 may block if @ref is in the process of switching to atomic
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* mode. If the caller ensures that @ref is not in the process of
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* switching to atomic mode, this function can be called from any context.
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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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unsigned long flags;
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spin_lock_irqsave(&percpu_ref_switch_lock, flags);
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ref->force_atomic = true;
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__percpu_ref_switch_mode(ref, confirm_switch);
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spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
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}
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EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic);
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/**
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* percpu_ref_switch_to_atomic_sync - switch a percpu_ref to atomic mode
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* @ref: percpu_ref to switch to atomic mode
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*
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* Schedule switching the ref to atomic mode, and wait for the
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* switch to complete. Caller must ensure that no other thread
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* will switch back to percpu mode.
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*/
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void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
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{
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percpu_ref_switch_to_atomic(ref, NULL);
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wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
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}
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EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync);
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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 may block if @ref is in the process of switching to atomic
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* mode. If the caller ensures that @ref is not in the process of
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* switching to atomic mode, this function can be called from any context.
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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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unsigned long flags;
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spin_lock_irqsave(&percpu_ref_switch_lock, flags);
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ref->force_atomic = false;
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__percpu_ref_switch_mode(ref, NULL);
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spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
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}
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EXPORT_SYMBOL_GPL(percpu_ref_switch_to_percpu);
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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_to_atomic().
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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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unsigned long flags;
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spin_lock_irqsave(&percpu_ref_switch_lock, flags);
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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_mode(ref, confirm_kill);
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percpu_ref_put(ref);
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spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
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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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unsigned long flags;
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spin_lock_irqsave(&percpu_ref_switch_lock, flags);
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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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__percpu_ref_switch_mode(ref, NULL);
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spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
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}
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EXPORT_SYMBOL_GPL(percpu_ref_reinit);
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