Patch "dm thin metadata: THIN_MAX_CONCURRENT_LOCKS should be 6" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    dm thin metadata: THIN_MAX_CONCURRENT_LOCKS should be 6

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     dm-thin-metadata-thin_max_concurrent_locks-should-be-6.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 490ae017f54e55bde382d45ea24bddfb6d1a0aaf Mon Sep 17 00:00:00 2001
From: Dennis Yang <dennisyang@qnap.com>
Date: Tue, 12 Dec 2017 18:21:40 +0800
Subject: dm thin metadata: THIN_MAX_CONCURRENT_LOCKS should be 6

From: Dennis Yang <dennisyang@qnap.com>

commit 490ae017f54e55bde382d45ea24bddfb6d1a0aaf upstream.

For btree removal, there is a corner case that a single thread
could takes 6 locks which is more than THIN_MAX_CONCURRENT_LOCKS(5)
and leads to deadlock.

A btree removal might eventually call
rebalance_children()->rebalance3() to rebalance entries of three
neighbor child nodes when shadow_spine has already acquired two
write locks. In rebalance3(), it tries to shadow and acquire the
write locks of all three child nodes. However, shadowing a child
node requires acquiring a read lock of the original child node and
a write lock of the new block. Although the read lock will be
released after block shadowing, shadowing the third child node
in rebalance3() could still take the sixth lock.
(2 write locks for shadow_spine +
 2 write locks for the first two child nodes's shadow +
 1 write lock for the last child node's shadow +
 1 read lock for the last child node)

Signed-off-by: Dennis Yang <dennisyang@qnap.com>
Acked-by: Joe Thornber <thornber@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/md/dm-thin-metadata.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/drivers/md/dm-thin-metadata.c
+++ b/drivers/md/dm-thin-metadata.c
@@ -81,10 +81,14 @@
 #define SECTOR_TO_BLOCK_SHIFT 3
 
 /*
+ * For btree insert:
  *  3 for btree insert +
  *  2 for btree lookup used within space map
+ * For btree remove:
+ *  2 for shadow spine +
+ *  4 for rebalance 3 child node
  */
-#define THIN_MAX_CONCURRENT_LOCKS 5
+#define THIN_MAX_CONCURRENT_LOCKS 6
 
 /* This should be plenty */
 #define SPACE_MAP_ROOT_SIZE 128


Patches currently in stable-queue which might be from dennisyang@qnap.com are

queue-4.4/dm-thin-metadata-thin_max_concurrent_locks-should-be-6.patch