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percpu-refcount: implement percpu_ref_reinit() and percpu_ref_is_zero()

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Now that explicit invocation of percpu_ref_exit() is necessary to free
the percpu counter, we can implement percpu_ref_reinit() which
reinitializes a released percpu_ref.  This can be used implement
scalable gating switch which can be drained and then re-opened without
worrying about memory allocation failures.

percpu_ref_is_zero() is added to be used in a sanity check in
percpu_ref_exit().  As this function will be useful for other purposes
too, make it a public interface.

v2: Use smp_read_barrier_depends() instead of smp_load_acquire().  We
    only need data dep barrier and smp_load_acquire() is stronger and
    heavier on some archs.  Spotted by Lai Jiangshan.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
This commit is contained in:
Tejun Heo 2014-06-28 08:10:14 -04:00
parent 9a1049da9b
commit 2d7227828e
2 changed files with 54 additions and 0 deletions
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35
lib/percpu-refcount.c
View file

@ -60,6 +60,41 @@ int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release)
}
EXPORT_SYMBOL_GPL(percpu_ref_init);
/**
* 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(). @ref must have been initialized successfully, killed
* 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)
{
unsigned __percpu *pcpu_count = pcpu_count_ptr(ref);
int cpu;
BUG_ON(!pcpu_count);
WARN_ON(!percpu_ref_is_zero(ref));
atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
/*
* Restore per-cpu operation. smp_store_release() is paired with
* smp_read_barrier_depends() in __pcpu_ref_alive() and guarantees
* that the zeroing is visible to all percpu accesses which can see
* the following PCPU_REF_DEAD clearing.
*/
for_each_possible_cpu(cpu)
*per_cpu_ptr(pcpu_count, cpu) = 0;
smp_store_release(&ref->pcpu_count_ptr,
ref->pcpu_count_ptr & ~PCPU_REF_DEAD);
}
EXPORT_SYMBOL_GPL(percpu_ref_reinit);
/**
* percpu_ref_exit - undo percpu_ref_init()
* @ref: percpu_ref to exit