[md PATCH 13/14] md: close a race with setting mddev->in_sync

NeilBrown
2017-02-16T04:39:03Z

If ->in_sync is being set just as md_write_start() is being called,
it is possible that set_in_sync() won't see the elevated
->writes_pending, and md_write_start() won't see the set ->in_sync.

To close this race, re-test ->writes_pending after setting ->in_sync,
and add memory barriers to ensure the increment of ->writes_pending
will be seen by the time of this second test, or the new ->in_sync
will be seen by md_write_start().

Add a spinlock to array_state_show() to ensure this temporary
instability is never visible from userspace.

Signed-off-by: NeilBrown <neilb@suse.com>
---
drivers/md/md.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 1d5fdc4a659d..f856c95ee7d5 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -2258,6 +2258,10 @@ static bool set_in_sync(struct mddev *mddev)
if (atomic_read(&mddev->writes_pending) == 0) {
if (mddev->in_sync == 0) {
mddev->in_sync = 1;
+ smp_mb();
+ if (atomic_read(&mddev->writes_pending))
+ /* lost a race with md_write_start() */
+ mddev->in_sync = 0;
set_bit(MD_SB_CHANGE_CLEAN, &mddev->sb_flags);
sysfs_notify_dirent_safe(mddev->sysfs_state);
}
@@ -3938,6 +3942,7 @@ array_state_show(struct mddev *mddev, char *page)
st = read_auto;
break;
case 0:
+ spin_lock(&mddev->lock);
if (test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags))
st = write_pending;
else if (mddev->in_sync)
@@ -3946,6 +3951,7 @@ array_state_show(struct mddev *mddev, char *page)
st = active_idle;
else
st = active;
+ spin_unlock(&mddev->lock);
}
else {
if (list_empty(&mddev->disks) &&
@@ -7769,6 +7775,7 @@ void md_write_start(struct mddev *mddev, struct bio *bi)
did_change = 1;
}
atomic_inc(&mddev->writes_pending);
+ smp_mb(); /* Match smp_mb in set_in_sync() */
if (mddev->safemode == 1)
mddev->safemode = 0;
if (mddev->in_sync) {