[PATCH] cgroup, writeback: add smp_mb() to inode_prepare_wbs_switch()

[Roman Gushchin]

Add a memory barrier between incrementing isw_nr_in_flight
and checking the sb's SB_ACTIVE flag and grabbing an inode in
inode_prepare_wbs_switch(). It's required to prevent grabbing
an inode before incrementing isw_nr_in_flight, otherwise
0 can be obtained as isw_nr_in_flight in cgroup_writeback_umount()
and isw_wq will not be flushed, potentially leading to a memory
corruption.

Added smp_mb() will work in pair with smp_mb() in
cgroup_writeback_umount().

Suggested-by: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Roman Gushchin <guro@xxxxxx>
---
 fs/fs-writeback.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 545fce68e919..6332b86ca4ed 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -513,6 +513,14 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
 static bool inode_prepare_wbs_switch(struct inode *inode,
 				     struct bdi_writeback *new_wb)
 {
+	/*
+	 * Paired with smp_mb() in cgroup_writeback_umount().
+	 * isw_nr_in_flight must be increased before checking SB_ACTIVE and
+	 * grabbing an inode, otherwise isw_nr_in_flight can be observed as 0
+	 * in cgroup_writeback_umount() and the isw_wq will be not flushed.
+	 */
+	smp_mb();
+
 	/* while holding I_WB_SWITCH, no one else can update the association */
 	spin_lock(&inode->i_lock);
 	if (!(inode->i_sb->s_flags & SB_ACTIVE) ||
-- 
2.31.1