From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roman Gushchin Subject: [PATCH v9 2/8] writeback, cgroup: add smp_mb() to cgroup_writeback_umount() Date: Tue, 8 Jun 2021 16:02:19 -0700 Message-ID: <20210608230225.2078447-3-guro@fb.com> References: <20210608230225.2078447-1-guro@fb.com> Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fb.com; h=from : to : cc : subject : date : message-id : in-reply-to : references : mime-version : content-transfer-encoding : content-type; s=facebook; bh=mfav6jzNKGWoxXgqLsimDL5+HxKJWmRmET2uHflSm9w=; b=OjqfaN2Os6DxSL7HmOn4jDtij1QwyZYPxFDhWgbljrVVB85ZPP7F2i1BRuaLZ7Rrp80u F9+mfkNCMARlB+MSC3rh4vz7UjDseYvOEtUZKn8LwPjakhCg/wlUY8oB9jdWmd3tnEb2 i0qc72fz4JnRtBJRCP8a4Pp2Nog77WtweDM= In-Reply-To: <20210608230225.2078447-1-guro@fb.com> List-ID: Content-Type: text/plain; charset="us-ascii" To: Andrew Morton , Tejun Heo Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Alexander Viro , Jan Kara , Dennis Zhou , Dave Chinner , cgroups@vger.kernel.org, Roman Gushchin 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 Signed-off-by: Roman Gushchin Reviewed-by: Jan Kara --- fs/fs-writeback.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 7d2891d7ac12..b6fc13a4962d 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_i= d, 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 --=20 2.31.1