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From: Greg KH on 11 Aug 2010 20:50
2.6.32-stable review patch. If anyone has any objections, please let us know.

------------------

From: NeilBrown <neilb(a)suse.de>

commit 51e9ac77035a3dfcb6fc0a88a0d80b6f99b5edb1 upstream.

If the 'bio_split' path in raid10-read is used while
resync/recovery is happening it is possible to deadlock.
Fix this be elevating ->nr_waiting for the duration of both
parts of the split request.

This fixes a bug that has been present since 2.6.22
but has only started manifesting recently for unknown reasons.
It is suitable for and -stable since then.

Reported-by: Justin Bronder <jsbronder(a)gentoo.org>
Tested-by: Justin Bronder <jsbronder(a)gentoo.org>
Signed-off-by: NeilBrown <neilb(a)suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh(a)suse.de>

---
drivers/md/raid10.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -824,11 +824,29 @@ static int make_request(struct request_q
*/
bp = bio_split(bio,
chunk_sects - (bio->bi_sector & (chunk_sects - 1)) );
+
+ /* Each of these 'make_request' calls will call 'wait_barrier'.
+ * If the first succeeds but the second blocks due to the resync
+ * thread raising the barrier, we will deadlock because the
+ * IO to the underlying device will be queued in generic_make_request
+ * and will never complete, so will never reduce nr_pending.
+ * So increment nr_waiting here so no new raise_barriers will
+ * succeed, and so the second wait_barrier cannot block.
+ */
+ spin_lock_irq(&conf->resync_lock);
+ conf->nr_waiting++;
+ spin_unlock_irq(&conf->resync_lock);
+
if (make_request(q, &bp->bio1))
generic_make_request(&bp->bio1);
if (make_request(q, &bp->bio2))
generic_make_request(&bp->bio2);

+ spin_lock_irq(&conf->resync_lock);
+ conf->nr_waiting--;
+ wake_up(&conf->wait_barrier);
+ spin_unlock_irq(&conf->resync_lock);
+
bio_pair_release(bp);
return 0;
bad_map:


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