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Date:	Thu, 19 Jun 2014 11:01:26 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<cl@...ux-foundation.org>, <kmo@...erainc.com>,
	<linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 6/6] percpu-refcount: implement percpu_ref_reinit()
 and percpu_ref_is_zero()

On 06/19/2014 10:20 AM, Tejun Heo wrote:
> 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@...nel.org>
> Cc: Kent Overstreet <kmo@...erainc.com>
> Cc: Christoph Lameter <cl@...ux-foundation.org>
> Cc: Lai Jiangshan <laijs@...fujitsu.com>
> ---
>  include/linux/percpu-refcount.h |   19 +++++++++++++++++++
>  lib/percpu-refcount.c           |   35 +++++++++++++++++++++++++++++++++++
>  2 files changed, 54 insertions(+)
> 
> --- a/include/linux/percpu-refcount.h
> +++ b/include/linux/percpu-refcount.h
> @@ -67,6 +67,7 @@ struct percpu_ref {
>  
>  int __must_check percpu_ref_init(struct percpu_ref *ref,
>  				 percpu_ref_func_t *release);
> +void percpu_ref_reinit(struct percpu_ref *ref);
>  void percpu_ref_exit(struct percpu_ref *ref);
>  void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
>  				 percpu_ref_func_t *confirm_kill);
> @@ -99,6 +100,9 @@ static inline bool __pcpu_ref_alive(stru
>  {
>  	unsigned long pcpu_ptr = ACCESS_ONCE(ref->pcpu_count_ptr);
>  
> +	/* paired with smp_store_release() in percpu_ref_reinit() */
> +	smp_read_barrier_depends();
> +
>  	if (unlikely(pcpu_ptr & PCPU_REF_DEAD))
>  		return false;
>  
> @@ -206,4 +210,19 @@ static inline void percpu_ref_put(struct
>  	rcu_read_unlock_sched();
>  }
>  
> +/**
> + * percpu_ref_is_zero - test whether a percpu refcount reached zero
> + * @ref: percpu_ref to test
> + *
> + * Returns %true if @ref reached zero.
> + */
> +static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
> +{
> +	unsigned __percpu *pcpu_count;
> +
> +	if (__pcpu_ref_alive(ref, &pcpu_count))
> +		return false;
> +	return !atomic_read(&ref->count);
> +}
> +
>  #endif
> --- a/lib/percpu-refcount.c
> +++ b/lib/percpu-refcount.c
> @@ -61,6 +61,41 @@ int percpu_ref_init(struct percpu_ref *r
>  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_load_acquire() in __pcpu_ref_alive() and guarantees that the

s/smp_load_acquire()/smp_read_barrier_depends()/

s/smp_store_release()/smp_mb()/  if you accept my next comment.

> +	 * 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);

I think it would be better if smp_mb() is used.

it is documented that smp_read_barrier_depends() and smp_mb() are paired.
Not smp_read_barrier_depends() and smp_store_release().

> +}
> +EXPORT_SYMBOL_GPL(percpu_ref_reinit);
> +
> +/**
>   * percpu_ref_exit - undo percpu_ref_init()
>   * @ref: percpu_ref to exit
>   *
> --
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