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

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Message-ID: <f70cae75-d866-f70d-1fea-4213b0fb8fef@huaweicloud.com>
Date:   Thu, 12 Jan 2023 14:17:27 +0800
From:   Hou Tao <houtao@...weicloud.com>
To:     Jingbo Xu <jefflexu@...ux.alibaba.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()

Hi,

On 1/12/2023 12:34 PM, Jingbo Xu wrote:
>
> 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.
Good idea. Just find the presence of clear_and_wake_up_bit(). Will do in v3 for
both patch 1 & patch 2.
>