• Ming Lei's avatar
    block: loop: don't hold lo_ctl_mutex in lo_open · f8933667
    Ming Lei authored
    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: default avatarChristoph Hellwig <hch@lst.de>
    Tested-by: default avatarJarod Wilson <jarod@redhat.com>
    Acked-by: default avatarJarod Wilson <jarod@redhat.com>
    Signed-off-by: default avatarMing Lei <ming.lei@canonical.com>
    Signed-off-by: default avatarJens Axboe <axboe@fb.com>
    f8933667
loop.h 2.25 KB