linux-kernel - [100/151] dm snapshot: only take lock for statustype info not table

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Date:	Wed, 16 Dec 2009 19:56:37 -0800
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Mikulas Patocka <mpatocka@...hat.com>,
	Alasdair G Kergon <agk@...hat.com>
Subject: [100/151] dm snapshot: only take lock for statustype info not table

2.6.32-stable review patch.  If anyone has any objections, please let us know.

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

From: Mikulas Patocka <mpatocka@...hat.com>

commit 94e76572b5dd37b1f0f4b3742ee8a565daead932 upstream.

Take snapshot lock only for STATUSTYPE_INFO, not STATUSTYPE_TABLE.

Commit 4c6fff445d7aa753957856278d4d93bcad6e2c14
(dm-snapshot-lock-snapshot-while-supplying-status.patch)
introduced this use of the lock, but userspace applications using
libdevmapper have been found to request STATUSTYPE_TABLE while the device
is suspended and the lock is already held, leading to deadlock.  Since
the lock is not necessary in this case, don't try to take it.

Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
Signed-off-by: Alasdair G Kergon <agk@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 drivers/md/dm-snap.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -1152,10 +1152,11 @@ static int snapshot_status(struct dm_tar
 	unsigned sz = 0;
 	struct dm_snapshot *snap = ti->private;
 
-	down_write(&snap->lock);
-
 	switch (type) {
 	case STATUSTYPE_INFO:
+
+		down_write(&snap->lock);
+
 		if (!snap->valid)
 			DMEMIT("Invalid");
 		else {
@@ -1171,6 +1172,9 @@ static int snapshot_status(struct dm_tar
 			else
 				DMEMIT("Unknown");
 		}
+
+		up_write(&snap->lock);
+
 		break;
 
 	case STATUSTYPE_TABLE:
@@ -1185,8 +1189,6 @@ static int snapshot_status(struct dm_tar
 		break;
 	}
 
-	up_write(&snap->lock);
-
 	return 0;
 }
 


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