On Sun, Jun 24, 2018 at 04:33:21PM +0000, Bart Van Assche wrote: > On Sun, 2018-06-24 at 18:16 +0800, Ming Lei wrote: > > Now I am revisiting 'TAG_SHARED in restart' again for the long delay issue > > of SCSI LUN probe. And found there is one bug in blk_mq_mark_tag_wait(): > > > > - hctx->dispatch_wait is added to wait queue by holding hctx->lock and > > the wait queue's lock > > > > - no hctx->lock is held when removing hctx->dispatch_wait from wait > > queue. > > > > - so the two actions(add, remove) may happen meantime since > > hctx->dispatch_wait can be added to different wait queues. > > > > Turns out this issue can be observed easily by applying the patches[2], > > which is for removing 'TAG_SHARED in restart', then run simple shared-tag > > null_blk test[4]. > > > > But if the hctx->lock is held in blk_mq_dispatch_wake(), as done in > > patch [3], there isn't such issue at all, so it shows this issue is > > related with hctx->lock, and adding/removing hctx->dispatch_wait to > > wait queue. But the way of holding hctx->lock in irq context may not > > be one accepted solution, since it has been avoided from the beginning > > of blk-mq. > > > > So does anyone have better ideas for this issue? > > > > So far, follows what I thought of: > > > > 1) fix the mechanism of blk_mq_mark_tag_wait(), and removing > > 'TAG_SHARED in restart', then we can fix the long delay issue of > > SCSI LUN probe, meantime performance can got improved, as I observed, > > this way may improve IOPS by 20~30% in multi-LUN scsi_debug test. > > But the issue is how to fix? > > > > 2) keep 'TAG_SHARED in restart' and let it cover the issue of > > blk_mq_mark_tag_wait() as now, then try to improve 'TAG_SHARED in restart' > > in another way, so that performance can be better, and synchronize_rcu() > > can be removed from blk_mq_del_queue_tag_set(), then SCSI LUN probe long > > delay can be fixed. I had wrote patches to do that last year. If anyone > > is interested, I may post it out. > > > > Or other ideas, any comments & ideas are welcome! > > Please have a look at [PATCH] blk-mq: Fix a race condition in blk_mq_mark_tag_wait(), > 16 Jan 2018 (https://www.mail-archive.com/linux-block@xxxxxxxxxxxxxxx/msg17474.html). Thanks for sharing it, looks I miss your findings. Your commit log describes the issue exactly, but unfortunately the patch isn't correct, because hctx->lock isn't held in the removing path of blk_mq_dispatch_wake(). Given 'hctx->dispatch_wait' may be added to different wait queues, it isn't enough to hold wait queue lock and hctx->lock in add path only. Otherwise, removing path can be seen as 'lockless' from the view point of add path. If you apply the patch[1] and the patch of '[PATCH] blk-mq: Fix a race condition in blk_mq_mark_tag_wait()', and run test script in [2], you will see that IO hang can still be triggered easily. I have cooked one patch of 'blk-mq: holding hctx->lock when removing hctx->dispatch_wait from wai'[3], which can fix the issue, but need to change all current pin_lock(hctx->lock) into spin_lock_irq() since blk_mq_dispatch_wake() is usually done in irq context. This kind of change might not be an accepted way, that is why I report it out and start the discussion. [1] https://github.com/ming1/linux/commit/36a0ff197531e02a955472059acfc436b8ed97e7 [2] http://people.redhat.com/minlei/tests/tools/null_blk_test-restart [2] https://github.com/ming1/linux/commit/cb7c822d663552da62479942458444c5081149a1 Thanks, Ming
