From: Zhengyuan Liu liuzhengyuan@kylinos.cn
commit c0e48f9dea9129aa11bec3ed13803bcc26e96e49 upstream.
There is a hang issue while using fio to do some basic test. The issue can be easily reproduced using the below script:
while true do fio --ioengine=io_uring -rw=write -bs=4k -numjobs=1 \ -size=1G -iodepth=64 -name=uring --filename=/dev/zero done
After several minutes (or more), fio would block at io_uring_enter->io_cqring_wait in order to waiting for previously committed sqes to be completed and can't return to user anymore until we send a SIGTERM to fio. After receiving SIGTERM, fio hangs at io_ring_ctx_wait_and_kill with a backtrace like this:
[54133.243816] Call Trace: [54133.243842] __schedule+0x3a0/0x790 [54133.243868] schedule+0x38/0xa0 [54133.243880] schedule_timeout+0x218/0x3b0 [54133.243891] ? sched_clock+0x9/0x10 [54133.243903] ? wait_for_completion+0xa3/0x130 [54133.243916] ? _raw_spin_unlock_irq+0x2c/0x40 [54133.243930] ? trace_hardirqs_on+0x3f/0xe0 [54133.243951] wait_for_completion+0xab/0x130 [54133.243962] ? wake_up_q+0x70/0x70 [54133.243984] io_ring_ctx_wait_and_kill+0xa0/0x1d0 [54133.243998] io_uring_release+0x20/0x30 [54133.244008] __fput+0xcf/0x270 [54133.244029] ____fput+0xe/0x10 [54133.244040] task_work_run+0x7f/0xa0 [54133.244056] do_exit+0x305/0xc40 [54133.244067] ? get_signal+0x13b/0xbd0 [54133.244088] do_group_exit+0x50/0xd0 [54133.244103] get_signal+0x18d/0xbd0 [54133.244112] ? _raw_spin_unlock_irqrestore+0x36/0x60 [54133.244142] do_signal+0x34/0x720 [54133.244171] ? exit_to_usermode_loop+0x7e/0x130 [54133.244190] exit_to_usermode_loop+0xc0/0x130 [54133.244209] do_syscall_64+0x16b/0x1d0 [54133.244221] entry_SYSCALL_64_after_hwframe+0x49/0xbe
The reason is that we had added a req to ctx->pending_async at the very end, but it didn't get a chance to be processed. How could this happen?
fio#cpu0 wq#cpu1
io_add_to_prev_work io_sq_wq_submit_work
atomic_read() <<< 1
atomic_dec_return() << 1->0 list_empty(); <<< true;
list_add_tail() atomic_read() << 0 or 1?
As atomic_ops.rst states, atomic_read does not guarantee that the runtime modification by any other thread is visible yet, so we must take care of that with a proper implicit or explicit memory barrier.
This issue was detected with the help of Jackie's liuyun01@kylinos.cn
Fixes: 31b515106428 ("io_uring: allow workqueue item to handle multiple buffered requests") Signed-off-by: Zhengyuan Liu liuzhengyuan@kylinos.cn Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/io_uring.c | 4 ++++ 1 file changed, 4 insertions(+)
--- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -1769,6 +1769,10 @@ static bool io_add_to_prev_work(struct a ret = true; spin_lock(&list->lock); list_add_tail(&req->list, &list->list); + /* + * Ensure we see a simultaneous modification from io_sq_wq_submit_work() + */ + smp_mb(); if (!atomic_read(&list->cnt)) { list_del_init(&req->list); ret = false;