From f8933667953e8e61bb6104f5ca88e32e85656a93 Mon Sep 17 00:00:00 2001 From: Ming Lei Date: Wed, 6 May 2015 12:26:23 +0800 Subject: block: loop: don't hold lo_ctl_mutex in lo_open The lo_ctl_mutex is held for running all ioctl handlers, and in some ioctl handlers, ioctl_by_bdev(BLKRRPART) is called for rereading partitions, which requires bd_mutex. So it is easy to cause failure because trylock(bd_mutex) may fail inside blkdev_reread_part(), and follows the lock context: blkid or other application: ->open() ->mutex_lock(bd_mutex) ->lo_open() ->mutex_lock(lo_ctl_mutex) losetup(set fd ioctl): ->mutex_lock(lo_ctl_mutex) ->ioctl_by_bdev(BLKRRPART) ->trylock(bd_mutex) This patch trys to eliminate the ABBA lock dependency by removing lo_ctl_mutext in lo_open() with the following approach: 1) make lo_refcnt as atomic_t and avoid acquiring lo_ctl_mutex in lo_open(): - for open vs. add/del loop, no any problem because of loop_index_mutex - freeze request queue during clr_fd, so I/O can't come until clearing fd is completed, like the effect of holding lo_ctl_mutex in lo_open - both open() and release() have been serialized by bd_mutex already 2) don't hold lo_ctl_mutex for decreasing/checking lo_refcnt in lo_release(), then lo_ctl_mutex is only required for the last release. Reviewed-by: Christoph Hellwig Tested-by: Jarod Wilson Acked-by: Jarod Wilson Signed-off-by: Ming Lei Signed-off-by: Jens Axboe --- drivers/block/loop.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'drivers/block/loop.h') diff --git a/drivers/block/loop.h b/drivers/block/loop.h index 49564edf5581..25e8997ed246 100644 --- a/drivers/block/loop.h +++ b/drivers/block/loop.h @@ -28,7 +28,7 @@ struct loop_func_table; struct loop_device { int lo_number; - int lo_refcnt; + atomic_t lo_refcnt; loff_t lo_offset; loff_t lo_sizelimit; int lo_flags; -- cgit v1.2.3