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343a72e5e3
Earlier commits in this series allow battery-powered systems to build their kernels with the default-disabled CONFIG_RCU_LAZY=y Kconfig option. This Kconfig option causes call_rcu() to delay its callbacks in order to batch callbacks. This means that a given RCU grace period covers more callbacks, thus reducing the number of grace periods, in turn reducing the amount of energy consumed, which increases battery lifetime which can be a very good thing. This is not a subtle effect: In some important use cases, the battery lifetime is increased by more than 10%. This CONFIG_RCU_LAZY=y option is available only for CPUs that offload callbacks, for example, CPUs mentioned in the rcu_nocbs kernel boot parameter passed to kernels built with CONFIG_RCU_NOCB_CPU=y. Delaying callbacks is normally not a problem because most callbacks do nothing but free memory. If the system is short on memory, a shrinker will kick all currently queued lazy callbacks out of their laziness, thus freeing their memory in short order. Similarly, the rcu_barrier() function, which blocks until all currently queued callbacks are invoked, will also kick lazy callbacks, thus enabling rcu_barrier() to complete in a timely manner. However, there are some cases where laziness is not a good option. For example, synchronize_rcu() invokes call_rcu(), and blocks until the newly queued callback is invoked. It would not be a good for synchronize_rcu() to block for ten seconds, even on an idle system. Therefore, synchronize_rcu() invokes call_rcu_hurry() instead of call_rcu(). The arrival of a non-lazy call_rcu_hurry() callback on a given CPU kicks any lazy callbacks that might be already queued on that CPU. After all, if there is going to be a grace period, all callbacks might as well get full benefit from it. Yes, this could be done the other way around by creating a call_rcu_lazy(), but earlier experience with this approach and feedback at the 2022 Linux Plumbers Conference shifted the approach to call_rcu() being lazy with call_rcu_hurry() for the few places where laziness is inappropriate. And another call_rcu() instance that cannot be lazy is the one on the percpu refcounter's "per-CPU to atomic switch" code path, which uses RCU when switching to atomic mode. The enqueued callback wakes up waiters waiting in the percpu_ref_switch_waitq. Allowing this callback to be lazy would result in unacceptable slowdowns for users of per-CPU refcounts, such as blk_pre_runtime_suspend(). Therefore, make __percpu_ref_switch_to_atomic() use call_rcu_hurry() in order to revert to the old behavior. [ paulmck: Apply s/call_rcu_flush/call_rcu_hurry/ feedback from Tejun Heo. ] Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> Acked-by: Tejun Heo <tj@kernel.org> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Cc: Dennis Zhou <dennis@kernel.org> Cc: Christoph Lameter <cl@linux.com> Cc: <linux-mm@kvack.org>
480 lines
15 KiB
C
480 lines
15 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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#define pr_fmt(fmt) "%s: " fmt, __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/slab.h>
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#include <linux/mm.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. @ref starts out in percpu mode with a refcount of 1 unless
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* @flags contains PERCPU_REF_INIT_ATOMIC or PERCPU_REF_INIT_DEAD. These flags
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* change the start state to atomic with the latter setting the initial refcount
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* to 0. See the 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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struct percpu_ref_data *data;
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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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data = kzalloc(sizeof(*ref->data), gfp);
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if (!data) {
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free_percpu((void __percpu *)ref->percpu_count_ptr);
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ref->percpu_count_ptr = 0;
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return -ENOMEM;
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}
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data->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
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data->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;
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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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data->allow_reinit = true;
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} else {
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start_count += PERCPU_COUNT_BIAS;
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}
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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(&data->count, start_count);
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data->release = release;
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data->confirm_switch = NULL;
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data->ref = ref;
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ref->data = data;
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return 0;
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}
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EXPORT_SYMBOL_GPL(percpu_ref_init);
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static 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->data && ref->data->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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/**
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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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struct percpu_ref_data *data = ref->data;
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unsigned long flags;
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__percpu_ref_exit(ref);
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if (!data)
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return;
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spin_lock_irqsave(&percpu_ref_switch_lock, flags);
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ref->percpu_count_ptr |= atomic_long_read(&ref->data->count) <<
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__PERCPU_REF_FLAG_BITS;
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ref->data = NULL;
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spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
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kfree(data);
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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_data *data = container_of(rcu,
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struct percpu_ref_data, rcu);
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struct percpu_ref *ref = data->ref;
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data->confirm_switch(ref);
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data->confirm_switch = NULL;
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wake_up_all(&percpu_ref_switch_waitq);
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if (!data->allow_reinit)
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__percpu_ref_exit(ref);
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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_data *data = container_of(rcu,
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struct percpu_ref_data, rcu);
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struct percpu_ref *ref = data->ref;
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unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
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static atomic_t underflows;
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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 %lu percpu %lu\n",
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atomic_long_read(&data->count), 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, &data->count);
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if (WARN_ONCE(atomic_long_read(&data->count) <= 0,
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"percpu ref (%ps) <= 0 (%ld) after switching to atomic",
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data->release, atomic_long_read(&data->count)) &&
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atomic_inc_return(&underflows) < 4) {
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pr_err("%s(): percpu_ref underflow", __func__);
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mem_dump_obj(data);
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}
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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->data->confirm_switch = confirm_switch ?:
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percpu_ref_noop_confirm_switch;
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percpu_ref_get(ref); /* put after confirmation */
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call_rcu_hurry(&ref->data->rcu,
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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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if (WARN_ON_ONCE(!ref->data->allow_reinit))
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return;
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atomic_long_add(PERCPU_COUNT_BIAS, &ref->data->count);
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/*
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* Restore per-cpu operation. smp_store_release() is paired
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* with READ_ONCE() in __ref_is_percpu() and guarantees that the
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* zeroing is visible to all percpu accesses which can see the
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* 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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struct percpu_ref_data *data = ref->data;
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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, !data->confirm_switch,
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percpu_ref_switch_lock);
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if (data->force_atomic || percpu_ref_is_dying(ref))
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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->data->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->data->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->data->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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* There are no implied RCU grace periods between kill and release.
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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(percpu_ref_is_dying(ref),
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"%s called more than once on %ps!", __func__,
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ref->data->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_is_zero - test whether a percpu refcount reached zero
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* @ref: percpu_ref to test
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*
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* Returns %true if @ref reached zero.
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*
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* This function is safe to call as long as @ref is between init and exit.
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*/
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bool percpu_ref_is_zero(struct percpu_ref *ref)
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{
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unsigned long __percpu *percpu_count;
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unsigned long count, flags;
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if (__ref_is_percpu(ref, &percpu_count))
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return false;
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/* protect us from being destroyed */
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spin_lock_irqsave(&percpu_ref_switch_lock, flags);
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if (ref->data)
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count = atomic_long_read(&ref->data->count);
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else
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count = ref->percpu_count_ptr >> __PERCPU_REF_FLAG_BITS;
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|
spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
|
|
|
|
return count == 0;
|
|
}
|
|
EXPORT_SYMBOL_GPL(percpu_ref_is_zero);
|
|
|
|
/**
|
|
* percpu_ref_reinit - re-initialize a percpu refcount
|
|
* @ref: perpcu_ref to re-initialize
|
|
*
|
|
* Re-initialize @ref so that it's in the same state as when it finished
|
|
* percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been
|
|
* initialized successfully and reached 0 but not exited.
|
|
*
|
|
* Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
|
|
* this function is in progress.
|
|
*/
|
|
void percpu_ref_reinit(struct percpu_ref *ref)
|
|
{
|
|
WARN_ON_ONCE(!percpu_ref_is_zero(ref));
|
|
|
|
percpu_ref_resurrect(ref);
|
|
}
|
|
EXPORT_SYMBOL_GPL(percpu_ref_reinit);
|
|
|
|
/**
|
|
* percpu_ref_resurrect - modify a percpu refcount from dead to live
|
|
* @ref: perpcu_ref to resurrect
|
|
*
|
|
* Modify @ref so that it's in the same state as before percpu_ref_kill() was
|
|
* called. @ref must be dead but must not yet have exited.
|
|
*
|
|
* If @ref->release() frees @ref then the caller is responsible for
|
|
* guaranteeing that @ref->release() does not get called while this
|
|
* function is in progress.
|
|
*
|
|
* Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
|
|
* this function is in progress.
|
|
*/
|
|
void percpu_ref_resurrect(struct percpu_ref *ref)
|
|
{
|
|
unsigned long __percpu *percpu_count;
|
|
unsigned long flags;
|
|
|
|
spin_lock_irqsave(&percpu_ref_switch_lock, flags);
|
|
|
|
WARN_ON_ONCE(!percpu_ref_is_dying(ref));
|
|
WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));
|
|
|
|
ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
|
|
percpu_ref_get(ref);
|
|
__percpu_ref_switch_mode(ref, NULL);
|
|
|
|
spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
|
|
}
|
|
EXPORT_SYMBOL_GPL(percpu_ref_resurrect);
|