on 1/11/2024 11:54 PM, Ming Lei wrote: > In blk_mq_mark_tag_wait(), __add_wait_queue() may be re-ordered > with the following blk_mq_get_driver_tag() in case of getting driver > tag failure. > > Then in __sbitmap_queue_wake_up(), waitqueue_active() may not observe > the added waiter in blk_mq_mark_tag_wait() and wake up nothing, meantime > blk_mq_mark_tag_wait() can't get driver tag successfully. > > This issue can be reproduced by running the following test in loop, and > fio hang can be observed in < 30min when running it on my test VM > in laptop. > > modprobe -r scsi_debug > modprobe scsi_debug delay=0 dev_size_mb=4096 max_queue=1 host_max_queue=1 submit_queues=4 > dev=`ls -d /sys/bus/pseudo/drivers/scsi_debug/adapter*/host*/target*/*/block/* | head -1 | xargs basename` > fio --filename=/dev/"$dev" --direct=1 --rw=randrw --bs=4k --iodepth=1 \ > --runtime=100 --numjobs=40 --time_based --name=test \ > --ioengine=libaio > > Fix the issue by adding one explicit barrier in blk_mq_mark_tag_wait(), which > is just fine in case of running out of tag. > > Apply the same pattern in blk_mq_get_tag() which should have same risk. > > Reported-by: Changhui Zhong <czhong@xxxxxxxxxx> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> > --- > BTW, Changhui is planning to upstream the test case to blktests. > > block/blk-mq-tag.c | 19 +++++++++++++++++++ > block/blk-mq.c | 16 ++++++++++++++++ > 2 files changed, 35 insertions(+) > > diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c > index cc57e2dd9a0b..29f77cae8eb2 100644 > --- a/block/blk-mq-tag.c > +++ b/block/blk-mq-tag.c > @@ -179,6 +179,25 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data) > > sbitmap_prepare_to_wait(bt, ws, &wait, TASK_UNINTERRUPTIBLE); > > + /* > + * Add one explicit barrier since __blk_mq_get_tag() may not > + * imply barrier in case of failure. > + * > + * Order adding us to wait queue and the following allocating > + * tag in __blk_mq_get_tag(). > + * > + * The pair is the one implied in sbitmap_queue_wake_up() > + * which orders clearing sbitmap tag bits and > + * waitqueue_active() in __sbitmap_queue_wake_up(), since > + * waitqueue_active() is lockless > + * > + * Otherwise, re-order of adding wait queue and getting tag > + * may cause __sbitmap_queue_wake_up() to wake up nothing > + * because the waitqueue_active() may not observe us in wait > + * queue. > + */ > + smp_mb(); > + Hi Ming, thanks for the fix. I'm not sure if we should explicitly imply a memory barrier here as prepare_to_wait variants normally imply a general memory barrier (see section "SLEEP AND WAKE-UP FUNCTIONS " in [1]). Wish this helps! [1] https://www.kernel.org/doc/Documentation/memory-barriers.txt > tag = __blk_mq_get_tag(data, bt); > if (tag != BLK_MQ_NO_TAG) > break; > diff --git a/block/blk-mq.c b/block/blk-mq.c > index fb29ff5cc281..54545a4792bf 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -1847,6 +1847,22 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx *hctx, > wait->flags &= ~WQ_FLAG_EXCLUSIVE; > __add_wait_queue(wq, wait); > > + /* > + * Add one explicit barrier since blk_mq_get_driver_tag() may > + * not imply barrier in case of failure. > + * > + * Order adding us to wait queue and allocating driver tag. > + * > + * The pair is the one implied in sbitmap_queue_wake_up() which > + * orders clearing sbitmap tag bits and waitqueue_active() in > + * __sbitmap_queue_wake_up(), since waitqueue_active() is lockless > + * > + * Otherwise, re-order of adding wait queue and getting driver tag > + * may cause __sbitmap_queue_wake_up() to wake up nothing because > + * the waitqueue_active() may not observe us in wait queue. > + */ > + smp_mb(); > + > /* > * It's possible that a tag was freed in the window between the > * allocation failure and adding the hardware queue to the wait >
