linux-kernel - Re: [PATCH v2 1/2] percpu_ref: percpu_ref_tryget

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Date:   Thu, 21 Oct 2021 10:01:24 -0400
From:   Dennis Zhou <dennis@...nel.org>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     linux-block@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2 1/2] percpu_ref: percpu_ref_tryget_live() version
 holding RCU

Hello,

On Thu, Oct 21, 2021 at 02:30:51PM +0100, Pavel Begunkov wrote:
> Add percpu_ref_tryget_live_rcu(), which is a version of
> percpu_ref_tryget_live() but the user is responsible for enclosing it in
> a RCU read lock section.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  include/linux/percpu-refcount.h | 33 +++++++++++++++++++++++----------
>  1 file changed, 23 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
> index ae16a9856305..b31d3f3312ce 100644
> --- a/include/linux/percpu-refcount.h
> +++ b/include/linux/percpu-refcount.h
> @@ -266,6 +266,28 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref)
>  	return percpu_ref_tryget_many(ref, 1);
>  }
>  
> +/**
> + * percpu_ref_tryget_live_rcu - same as percpu_ref_tryget_live() but the
> + * caller is responsible for taking RCU.
> + *
> + * This function is safe to call as long as @ref is between init and exit.
> + */
> +static inline bool percpu_ref_tryget_live_rcu(struct percpu_ref *ref)
> +{
> +	unsigned long __percpu *percpu_count;
> +	bool ret = false;
> +
> +	WARN_ON_ONCE(!rcu_read_lock_held());
> +
> +	if (likely(__ref_is_percpu(ref, &percpu_count))) {
> +		this_cpu_inc(*percpu_count);
> +		ret = true;
> +	} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
> +		ret = atomic_long_inc_not_zero(&ref->data->count);
> +	}
> +	return ret;
> +}
> +
>  /**
>   * percpu_ref_tryget_live - try to increment a live percpu refcount
>   * @ref: percpu_ref to try-get

Nit: it's dumb convention at this point, but do you mind copying this
guy up. I like consistency.

> @@ -283,20 +305,11 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref)
>   */
>  static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
>  {
> -	unsigned long __percpu *percpu_count;
>  	bool ret = false;
>  
>  	rcu_read_lock();
> -
> -	if (__ref_is_percpu(ref, &percpu_count)) {
> -		this_cpu_inc(*percpu_count);
> -		ret = true;
> -	} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
> -		ret = atomic_long_inc_not_zero(&ref->data->count);
> -	}
> -
> +	ret = percpu_ref_tryget_live_rcu(ref);
>  	rcu_read_unlock();
> -
>  	return ret;
>  }
>  
> -- 
> 2.33.1
> 

Currently I'm not carrying anything and I don't expect any percpu_ref
work to come in. Jens, feel free to pick this up.

Acked-by: Dennis Zhou <dennis@...nel.org>

Thanks,
Dennis