Patch "btrfs: Fix a deadlock in btrfs_dev_replace_finishing()" has been added to the 3.16-stable tree

[g***@linuxfoundation.org]

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

btrfs: Fix a deadlock in btrfs_dev_replace_finishing()

to the 3.16-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:
btrfs-fix-a-deadlock-in-btrfs_dev_replace_finishing.patch
and it can be found in the queue-3.16 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,

From 12b894cb288d57292b01cf158177b6d5c89a6272 Mon Sep 17 00:00:00 2001

From: Qu Wenruo <***@cn.fujitsu.com>
Date: Wed, 20 Aug 2014 16:10:15 +0800
Subject: btrfs: Fix a deadlock in btrfs_dev_replace_finishing()

From: Qu Wenruo <***@cn.fujitsu.com>

commit 12b894cb288d57292b01cf158177b6d5c89a6272 upstream.

btrfs-transacion:5657
[stack snip]
btrfs_bio_map()
btrfs_bio_counter_inc_blocked()
percpu_counter_inc(&fs_info->bio_counter) ###bio_counter > 0(A)
__btrfs_bio_map()
btrfs_dev_replace_lock()
mutex_lock(dev_replace->lock) ###wait mutex(B)

btrfs:32612
[stack snip]
btrfs_dev_replace_start()
btrfs_dev_replace_lock()
mutex_lock(dev_replace->lock) ###hold mutex(B)
btrfs_dev_replace_finishing()
btrfs_rm_dev_replace_blocked()
wait until percpu_counter_sum == 0 ###wait on bio_counter(A)

This bug can be triggered quite easily by the following test script:
http://pastebin.com/MQmb37Cy

This patch will fix the ABBA problem by calling
btrfs_dev_replace_unlock() before btrfs_rm_dev_replace_blocked().

The consistency of btrfs devices list and their superblocks is protected
by device_list_mutex, not btrfs_dev_replace_lock/unlock().
So it is safe the move btrfs_dev_replace_unlock() before
btrfs_rm_dev_replace_blocked().

Reported-by: Zhao Lei <***@cn.fujitsu.com>
Signed-off-by: Qu Wenruo <***@cn.fujitsu.com>
Cc: Stefan Behrens <***@giantdisaster.de>
Signed-off-by: Chris Mason <***@fb.com>
Signed-off-by: Greg Kroah-Hartman <***@linuxfoundation.org>

---
fs/btrfs/dev-replace.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -567,6 +567,8 @@ static int btrfs_dev_replace_finishing(s
btrfs_kobj_rm_device(fs_info, src_device);
btrfs_kobj_add_device(fs_info, tgt_device);

+ btrfs_dev_replace_unlock(dev_replace);
+
btrfs_rm_dev_replace_blocked(fs_info);

btrfs_rm_dev_replace_srcdev(fs_info, src_device);
@@ -580,7 +582,6 @@ static int btrfs_dev_replace_finishing(s
* superblock is scratched out so that it is no longer marked to
* belong to this filesystem.
*/
- btrfs_dev_replace_unlock(dev_replace);
mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
mutex_unlock(&root->fs_info->chunk_mutex);



Patches currently in stable-queue which might be from ***@cn.fujitsu.com are

queue-3.16/btrfs-fix-a-deadlock-in-btrfs_dev_replace_finishing.patch