linux-kernel - Re: [PATCH v2 2/2] fscache: Add the missing smp_mb__after_atomic() before wake_up

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Date:   Thu, 12 Jan 2023 12:34:30 +0800
From:   Jingbo Xu <jefflexu@...ux.alibaba.com>
To:     Hou Tao <houtao@...weicloud.com>, linux-cachefs@...hat.com
Cc:     David Howells <dhowells@...hat.com>,
        Jeff Layton <jlayton@...nel.org>, linux-erofs@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, houtao1@...wei.com
Subject: Re: [PATCH v2 2/2] fscache: Add the missing smp_mb__after_atomic()
 before wake_up_bit()



On 12/26/22 6:33 PM, Hou Tao wrote:
> From: Hou Tao <houtao1@...wei.com>
> 
> fscache_create_volume_work() uses wake_up_bit() to wake up the processes
> which are waiting for the completion of volume creation. According to
> comments in wake_up_bit() and waitqueue_active(), an extra smp_mb() is
> needed to guarantee the memory order between FSCACHE_VOLUME_CREATING
> flag and waitqueue_active() before invoking wake_up_bit().
> 
> Considering clear_bit_unlock() before wake_up_bit() is an atomic
> operation, use smp_mb__after_atomic() instead of smp_mb() to provide
> such guarantee.
> 
> Signed-off-by: Hou Tao <houtao1@...wei.com>
> ---
>  fs/fscache/volume.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/fs/fscache/volume.c b/fs/fscache/volume.c
> index fc3dd3bc851d..734d17f404e7 100644
> --- a/fs/fscache/volume.c
> +++ b/fs/fscache/volume.c
> @@ -281,6 +281,11 @@ static void fscache_create_volume_work(struct work_struct *work)
>  				 fscache_access_acquire_volume_end);
>  
>  	clear_bit_unlock(FSCACHE_VOLUME_CREATING, &volume->flags);
> +	/*
> +	 * Paired with barrier in wait_on_bit(). Check wake_up_bit() and
> +	 * waitqueue_active() for details.
> +	 */
> +	smp_mb__after_atomic();
>  	wake_up_bit(&volume->flags, FSCACHE_VOLUME_CREATING);
>  	fscache_put_volume(volume, fscache_volume_put_create_work);
>  }

LGTM.

Actually I'm thinking if clear_and_wake_up_bit() could be used here.
Ditto for patch 1.

-- 
Thanks,
Jingbo