dm: dont take i_mutex to change device size

No longer needlessly hold md->bdev->bd_inode->i_mutex when changing the
size of a DM device.  This additional locking is unnecessary because
i_size_write() is already protected by the existing critical section in
dm_swap_table().  DM already has a reference on md->bdev so the
associated bd_inode may be changed without lifetime concerns.

A negative side-effect of having held md->bdev->bd_inode->i_mutex was
that a concurrent DM device resize and flush (via fsync) would deadlock.
Dropping md->bdev->bd_inode->i_mutex eliminates this potential for
deadlock.  The following reproducer no longer deadlocks:
  https://www.redhat.com/archives/dm-devel/2009-July/msg00284.htmlSigned-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: stable@kernel.org

drivers/md/dm.c

@@ -1992,13 +1992,14 @@ static void event_callback(void *context)
 	wake_up(&md->eventq);
 }
 
+/*
+ * Protected by md->suspend_lock obtained by dm_swap_table().
+ */
 static void __set_size(struct mapped_device *md, sector_t size)
 {
 	set_capacity(md->disk, size);
 
-	mutex_lock(&md->bdev->bd_inode->i_mutex);
 	i_size_write(md->bdev->bd_inode, (loff_t)size << SECTOR_SHIFT);
-	mutex_unlock(&md->bdev->bd_inode->i_mutex);
 }
 
 /*