Patch "btrfs: fix lock dep warning move scratch super outside of chunk_mutex" has been added to the 4.5-stable tree

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

btrfs: fix lock dep warning move scratch super outside of chunk_mutex

to the 4.5-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-lock-dep-warning-move-scratch-super-outside-of-chunk_mutex.patch
and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Anand Jain <[email protected]>
Date: Tue, 12 Apr 2016 21:36:16 +0800
Subject: btrfs: fix lock dep warning move scratch super outside of chunk_mutex

From: Anand Jain <[email protected]>

commit 48b3b9d401ec86899a52003b37331190a35a81a6 upstream.

Move scratch super outside of the chunk lock to avoid below lockdep warning. The better place to scratch super is in the function btrfs_rm_dev_replace_free_srcdev() just before free_device, which is outside of the chunk lock as well.

To reproduce:
(fresh boot)
mkfs.btrfs -f -draid5 -mraid5 /dev/sdc /dev/sdd /dev/sde mount /dev/sdc /btrfs
dd if=/dev/zero of=/btrfs/tf1 bs=4096 count=100 (get devmgt from https://github.com/asj/devmgt.git)
devmgt detach /dev/sde
dd if=/dev/zero of=/btrfs/tf1 bs=4096 count=100 sync
btrfs replace start -Brf 3 /dev/sdf /btrfs <-- devmgt attach host7

======================================================
[ INFO: possible circular locking dependency detected ] 4.6.0-rc2asj+ #1 Not tainted
---------------------------------------------------

btrfs/2174 is trying to acquire lock:
(sb_writers){.+.+.+}, at:
[<ffffffff812449b4>] __sb_start_write+0xb4/0xf0

but task is already holding lock:
(&fs_info->chunk_mutex){+.+.+.}, at:
[<ffffffffa05c5f55>] btrfs_dev_replace_finishing+0x145/0x980 [btrfs]

which lock already depends on the new lock.

Chain exists of:
sb_writers --> &fs_devs->device_list_mutex --> &fs_info->chunk_mutex
Possible unsafe locking scenario:
CPU0				CPU1
----				----
lock(&fs_info->chunk_mutex);
				lock(&fs_devs->device_list_mutex);
				lock(&fs_info->chunk_mutex);

lock(sb_writers);

---
 fs/btrfs/volumes.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

@@ -1971,11 +1971,8 @@ void btrfs_rm_dev_replace_remove_srcdev(
 	if (srcdev->missing)
 		fs_devices->missing_devices--;
 
-	if (srcdev->writeable) {
+	if (srcdev->writeable)
 		fs_devices->rw_devices--;
-		/* zero out the old super if it is writable */
-		btrfs_scratch_superblocks(srcdev->bdev, srcdev->name->str);
-	}
 
 	if (srcdev->bdev)
 		fs_devices->open_devices--;

@@ -1986,6 +1983,10 @@ void btrfs_rm_dev_replace_free_srcdev(st
 {
 	struct btrfs_fs_devices *fs_devices = srcdev->fs_devices;
 
+	if (srcdev->writeable) {
+		/* zero out the old super if it is writable */
+		btrfs_scratch_superblocks(srcdev->bdev, srcdev->name->str);
+	}
 	call_rcu(&srcdev->rcu, free_device);
 
 	/*

Patches currently in stable-queue which might be from [email protected] are

queue-4.5/btrfs-pass-the-right-error-code-to-the-btrfs_std_error.patch queue-4.5/btrfs-fix-lock-dep-warning-move-scratch-super-outside-of-chunk_mutex.patch queue-4.5/btrfs-fix-lock-dep-warning-move-scratch-dev-out-of-device_list_mutex-and-uuid_mutex.patch