linux-kernel - [PATCH v8 2/8] writeback, cgroup: add smp_mb() to cgroup_writeback

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Message-ID: <20210608013123.1088882-3-guro@fb.com>
Date:   Mon, 7 Jun 2021 18:31:17 -0700
From:   Roman Gushchin <guro@...com>
To:     Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>
CC:     <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, Alexander Viro <viro@...iv.linux.org.uk>,
        Dennis Zhou <dennis@...nel.org>,
        Dave Chinner <dchinner@...hat.com>, <cgroups@...r.kernel.org>,
        Roman Gushchin <guro@...com>
Subject: [PATCH v8 2/8] writeback, cgroup: add smp_mb() to cgroup_writeback_umount()

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@...e.cz>
Signed-off-by: Roman Gushchin <guro@...com>
---
 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