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Subject: Re: [PATCH v2 2/2] fscache: Add the missing smp_mb__after_atomic()
 before wake_up_bit()
To:     David Howells <dhowells@redhat.com>
Cc:     linux-cachefs@redhat.com, Jeff Layton <jlayton@kernel.org>,
        linux-erofs@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        Jingbo Xu <jefflexu@linux.alibaba.com>, houtao1@huawei.com
References: <20221226103309.953112-3-houtao@huaweicloud.com>
 <20221226103309.953112-1-houtao@huaweicloud.com>
 <2431994.1673453386@warthog.procyon.org.uk>
From:   Hou Tao <houtao@huaweicloud.com>
Message-ID: <cd26e985-ce9a-6011-400a-9005376f47b5@huaweicloud.com>
Date:   Thu, 12 Jan 2023 09:07:32 +0800
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Hi,

On 1/12/2023 12:09 AM, David Howells wrote:
> Hou Tao <houtao@huaweicloud.com> wrote:
>
>> 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().
> What two values are you ordering?
>
> If we're using this to create a critical section, then yes, we would need a
> barrier to order the changes inside the critical section before changing the
> memory location that forms the lock - but this is not a critical section.
It is similar with patch #1. The smp_mb() is used for order between
volume->flags and wq->head.
> David
>