btrfs: fix a deadlock in btrfs_scan_one_device()

pathname resolution under a global mutex, taken on some paths in ->mount()
is a Bad Idea(tm) - think what happens if said pathname resolution triggers
automount of some btrfs instance and walks into attempt to grab the same
mutex.  Deadlock - we are waiting for daemon to finish walking the path,
daemon is waiting for us to release the mutex...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 2 insertions, 3 deletions

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index f4b839fd3c9d..96bb3c0a79b0 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -706,8 +706,6 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
         u64 devid;
         u64 transid;
 
-        mutex_lock(&uuid_mutex);
-
         flags |= FMODE_EXCL;
         bdev = blkdev_get_by_path(path, flags, holder);
 
@@ -716,6 +714,7 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
                 goto error;
         }
 
+        mutex_lock(&uuid_mutex);
         ret = set_blocksize(bdev, 4096);
         if (ret)
                 goto error_close;
@@ -737,9 +736,9 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
 
         brelse(bh);
 error_close:
+        mutex_unlock(&uuid_mutex);
         blkdev_put(bdev, flags);
 error:
-        mutex_unlock(&uuid_mutex);
         return ret;
 }