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Subject[PATCH 4.12 09/41] md: fix test in md_write_start()
4.12-stable review patch.  If anyone has any objections, please let me know.


From: NeilBrown <>

commit 81fe48e9aa00bdd509bd3c37a76d1132da6b9f09 upstream.

md_write_start() needs to clear the in_sync flag is it is set, or if
there might be a race with set_in_sync() such that the later will
set it very soon. In the later case it is sufficient to take the
spinlock to synchronize with set_in_sync(), and then set the flag
if needed.

The current test is incorrect.
It should be:
if "flag is set" or "race is possible"

"flag is set" is trivially "mddev->in_sync".
"race is possible" should be tested by "mddev->sync_checkers".

If sync_checkers is 0, then there can be no race. set_in_sync() will
wait in percpu_ref_switch_to_atomic_sync() for an RCU grace period,
and as md_write_start() holds the rcu_read_lock(), set_in_sync() will
be sure ot see the update to writes_pending.

If sync_checkers is > 0, there could be race. If md_write_start()
happened entirely between
if (!mddev->in_sync &&
percpu_ref_is_zero(&mddev->writes_pending)) {
mddev->in_sync = 1;
in set_in_sync(), then it would not see that is_sync had been set,
and set_in_sync() would not see that writes_pending had been

This bug means that in_sync is sometimes not set when it should be.
Consequently there is a small chance that the array will be marked as
"clean" when in fact it is inconsistent.

Fixes: 4ad23a976413 ("MD: use per-cpu counter for writes_pending")
Signed-off-by: NeilBrown <>
Signed-off-by: Shaohua Li <>
Signed-off-by: Greg Kroah-Hartman <>

drivers/md/md.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -7979,7 +7979,7 @@ bool md_write_start(struct mddev *mddev,
if (mddev->safemode == 1)
mddev->safemode = 0;
/* sync_checkers is always 0 when writes_pending is in per-cpu mode */
- if (mddev->in_sync || !mddev->sync_checkers) {
+ if (mddev->in_sync || mddev->sync_checkers) {
if (mddev->in_sync) {
mddev->in_sync = 0;

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