Re: [PATCH v8 2/8] writeback, cgroup: add smp_mb() to cgroup_writeback_umount()

From: Jan Kara <jack@suse.cz>
To: Roman Gushchin <guro@fb.com>
Cc: Jan Kara <jack@suse.cz>, Tejun Heo <tj@kernel.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, Alexander Viro <viro@zeniv.linux.org.uk>,
	Dennis Zhou <dennis@kernel.org>,
	Dave Chinner <dchinner@redhat.com>,
	cgroups@vger.kernel.org
Subject: Re: [PATCH v8 2/8] writeback, cgroup: add smp_mb() to cgroup_writeback_umount()
Date: Tue, 8 Jun 2021 10:43:48 +0200	[thread overview]
Message-ID: <20210608084348.GA5562@quack2.suse.cz> (raw)
In-Reply-To: <20210608013123.1088882-3-guro@fb.com>

On Mon 07-06-21 18:31:17, Roman Gushchin wrote:
> A full memory barrier is required between clearing SB_ACTIVE flag
> in generic_shutdown_super() and checking isw_nr_in_flight in
> cgroup_writeback_umount(), otherwise a new switch operation might
> be scheduled after atomic_read(&isw_nr_in_flight) returned 0.
> This would result in a non-flushed isw_wq, and a potential crash.
> 
> The problem hasn't yet been seen in the real life and was discovered
> by Jan Kara by looking into the code.
> 
> Suggested-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Roman Gushchin <guro@fb.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/fs-writeback.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index bd99890599e0..3564efcc4b78 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1000,6 +1000,12 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_id, unsigned long nr,
>   */
>  void cgroup_writeback_umount(void)
>  {
> +	/*
> +	 * SB_ACTIVE should be reliably cleared before checking
> +	 * isw_nr_in_flight, see generic_shutdown_super().
> +	 */
> +	smp_mb();
> +
>  	if (atomic_read(&isw_nr_in_flight)) {
>  		/*
>  		 * Use rcu_barrier() to wait for all pending callbacks to
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR