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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 <hch@lst.de>
Tested-by: Jarod Wilson <jarod@redhat.com>
Acked-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
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Documentation/workqueue.txt:
If there is dependency among multiple work items used
during memory reclaim, they should be queued to separate
wq each with WQ_MEM_RECLAIM.
Loop devices can be stacked, so we have to convert to per-device
workqueue. One example is Fedora live CD.
Fixes: b5dd2f6047ca108001328aac0e8588edd15f1778
Cc: stable@vger.kernel.org (v4.0)
Cc: Justin M. Forbes <jforbes@fedoraproject.org>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>
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Looks like we pull it in through other ways on x86, but we fail
on sparc:
In file included from drivers/block/cryptoloop.c:30:0:
drivers/block/loop.h:63:24: error: field 'tag_set' has incomplete type
struct blk_mq_tag_set tag_set;
Add the include to loop.h, kill it from loop.c.
Signed-off-by: Jens Axboe <axboe@fb.com>
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The conversion is a bit straightforward, and use work queue to
dispatch requests of loop block, and one big change is that requests
is submitted to backend file/device concurrently with work queue,
so throughput may get improved much. Given write requests over same
file are often run exclusively, so don't handle them concurrently for
avoiding extra context switch cost, possible lock contention and work
schedule cost. Also with blk-mq, there is opportunity to get loop I/O
merged before submitting to backend file/device.
In the following test:
- base: v3.19-rc2-2041231
- loop over file in ext4 file system on SSD disk
- bs: 4k, libaio, io depth: 64, O_DIRECT, num of jobs: 1
- throughput: IOPS
------------------------------------------------------
| | base | base with loop-mq | delta |
------------------------------------------------------
| randread | 1740 | 25318 | +1355%|
------------------------------------------------------
| read | 42196 | 51771 | +22.6%|
-----------------------------------------------------
| randwrite | 35709 | 34624 | -3% |
-----------------------------------------------------
| write | 39137 | 40326 | +3% |
-----------------------------------------------------
So loop-mq can improve throughput for both read and randread, meantime,
performance of write and randwrite isn't hurted basically.
Another benefit is that loop driver code gets simplified
much after blk-mq conversion, and the patch can be thought as
cleanup too.
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
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Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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