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

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Date:   Wed, 11 Jan 2023 16:09:46 +0000
From:   David Howells <dhowells@...hat.com>
To:     Hou Tao <houtao@...weicloud.com>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com,
        Jeff Layton <jlayton@...nel.org>, linux-erofs@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org,
        Jingbo Xu <jefflexu@...ux.alibaba.com>, houtao1@...wei.com
Subject: Re: [PATCH v2 2/2] fscache: Add the missing smp_mb__after_atomic() before wake_up_bit()

Hou Tao <houtao@...weicloud.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.

David