[PATCH 1/1] Set prev_flush_start and flush_bio in an atomic way

[Xiao Ni <xni@redhat.com>]

One customer reports a crash problem which causes by flush request. It triggers a warning
before crash.

        /* new request after previous flush is completed */
        if (ktime_after(req_start, mddev->prev_flush_start)) {
                WARN_ON(mddev->flush_bio);
                mddev->flush_bio = bio;
                bio = NULL;
        }

The WARN_ON is triggered. We use spin lock to protect prev_flush_start and flush_bio
in md_flush_request. But there is no lock protection in md_submit_flush_data. It can
set flush_bio to NULL first because of compiler reordering write instructions.

For example, flush bio1 sets flush bio to NULL first in md_submit_flush_data. An
interrupt or vmware causing an extended stall happen between updating flush_bio and
prev_flush_start. Because flush_bio is NULL, flush bio2 can get the lock and submit
to underlayer disks. Then flush bio1 updates prev_flush_start after the interrupt or
exteneded stall.

Then flush bio3 enters in md_flush_request. The start time req_start is behind
prev_flush_start. The flush_bio is not NULL(flush bio2 hasn't finished). So it can
trigger the WARN_ON now. Then it calls INIT_WORK again. INIT_WORK() will re-initialize
the list pointers in the work_struct, which then can result in a corrupted work list
and the work_struct queued a second time. With the work list corrupted, it can lead
in invalid work items being used and cause a crash in process_one_work.

We need to make sure only one flush bio can be handled at one same time. So add
spin lock in md_submit_flush_data to protect prev_flush_start and flush_bio in
an atomic way.

Reviewed-by: David Jeffery <djeffery@redhat.com>
Signed-off-by: Xiao Ni <xni@redhat.com>
---
 drivers/md/md.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index c42af46..2746d5c 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -639,8 +639,10 @@ static void md_submit_flush_data(struct work_struct *ws)
 	 * could wait for this and below md_handle_request could wait for those
 	 * bios because of suspend check
 	 */
+	spin_lock_irq(&mddev->lock);
 	mddev->prev_flush_start = mddev->start_flush;
 	mddev->flush_bio = NULL;
+	spin_unlock_irq(&mddev->lock);
 	wake_up(&mddev->sb_wait);
 
 	if (bio->bi_iter.bi_size == 0) {
-- 
2.7.5