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Date: Wed, 19 Feb 2020 09:49:41 +0800
From: Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
To: io-uring@...r.kernel.org
Cc: axboe@...nel.dk, joseph.qi@...ux.alibaba.com,
Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH v2] io_uring: fix poll_list race for
SETUP_IOPOLL|SETUP_SQPOLL
hi,
Cc ext4 mail list as well, in case someone runs into the same issue.
Regards,
Xiaoguang Wang
> After making ext4 support iopoll method:
> let ext4_file_operations's iopoll method be iomap_dio_iopoll(),
> we found fio can easily hang in fio_ioring_getevents() with below fio
> job:
> rm -f testfile; sync;
> sudo fio -name=fiotest -filename=testfile -iodepth=128 -thread
> -rw=write -ioengine=io_uring -hipri=1 -sqthread_poll=1 -direct=1
> -bs=4k -size=10G -numjobs=8 -runtime=2000 -group_reporting
> with IORING_SETUP_SQPOLL and IORING_SETUP_IOPOLL enabled.
>
> There are two issues that results in this hang, one reason is that
> when IORING_SETUP_SQPOLL and IORING_SETUP_IOPOLL are enabled, fio
> does not use io_uring_enter to get completed events, it relies on
> kernel io_sq_thread to poll for completed events.
>
> Another reason is that there is a race: when io_submit_sqes() in
> io_sq_thread() submits a batch of sqes, variable 'inflight' will
> record the number of submitted reqs, then io_sq_thread will poll for
> reqs which have been added to poll_list. But note, if some previous
> reqs have been punted to io worker, these reqs will won't be in
> poll_list timely. io_sq_thread() will only poll for a part of previous
> submitted reqs, and then find poll_list is empty, reset variable
> 'inflight' to be zero. If app just waits these deferred reqs and does
> not wake up io_sq_thread again, then hang happens.
>
> For app that entirely relies on io_sq_thread to poll completed requests,
> let io_iopoll_req_issued() wake up io_sq_thread properly when adding new
> element to poll_list.
>
> Fixes: 2b2ed9750fc9 ("io_uring: fix bad inflight accounting for SETUP_IOPOLL|SETUP_SQTHREAD")
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
>
> ---
> V2:
> simple code cleanups and add necessary comments.
> ---
> fs/io_uring.c | 72 ++++++++++++++++++++++++++++-----------------------
> 1 file changed, 40 insertions(+), 32 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 77f22c3da30f..b6d7c45d0d0d 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1793,6 +1793,9 @@ static void io_iopoll_req_issued(struct io_kiocb *req)
> list_add(&req->list, &ctx->poll_list);
> else
> list_add_tail(&req->list, &ctx->poll_list);
> +
> + if (ctx->flags & IORING_SETUP_SQPOLL && wq_has_sleeper(&ctx->sqo_wait))
> + wake_up(&ctx->sqo_wait);
> }
>
> static void io_file_put(struct io_submit_state *state)
> @@ -5011,9 +5014,9 @@ static int io_sq_thread(void *data)
> const struct cred *old_cred;
> mm_segment_t old_fs;
> DEFINE_WAIT(wait);
> - unsigned inflight;
> unsigned long timeout;
> - int ret;
> + int ret = 0;
> + bool needs_uring_lock = false;
>
> complete(&ctx->completions[1]);
>
> @@ -5021,39 +5024,21 @@ static int io_sq_thread(void *data)
> set_fs(USER_DS);
> old_cred = override_creds(ctx->creds);
>
> - ret = timeout = inflight = 0;
> + if (ctx->flags & IORING_SETUP_IOPOLL)
> + needs_uring_lock = true;
> + timeout = jiffies + ctx->sq_thread_idle;
> while (!kthread_should_park()) {
> unsigned int to_submit;
>
> - if (inflight) {
> + if (!list_empty(&ctx->poll_list)) {
> unsigned nr_events = 0;
>
> - if (ctx->flags & IORING_SETUP_IOPOLL) {
> - /*
> - * inflight is the count of the maximum possible
> - * entries we submitted, but it can be smaller
> - * if we dropped some of them. If we don't have
> - * poll entries available, then we know that we
> - * have nothing left to poll for. Reset the
> - * inflight count to zero in that case.
> - */
> - mutex_lock(&ctx->uring_lock);
> - if (!list_empty(&ctx->poll_list))
> - __io_iopoll_check(ctx, &nr_events, 0);
> - else
> - inflight = 0;
> - mutex_unlock(&ctx->uring_lock);
> - } else {
> - /*
> - * Normal IO, just pretend everything completed.
> - * We don't have to poll completions for that.
> - */
> - nr_events = inflight;
> - }
> -
> - inflight -= nr_events;
> - if (!inflight)
> + mutex_lock(&ctx->uring_lock);
> + if (!list_empty(&ctx->poll_list))
> + __io_iopoll_check(ctx, &nr_events, 0);
> + if (list_empty(&ctx->poll_list))
> timeout = jiffies + ctx->sq_thread_idle;
> + mutex_unlock(&ctx->uring_lock);
> }
>
> to_submit = io_sqring_entries(ctx);
> @@ -5070,7 +5055,7 @@ static int io_sq_thread(void *data)
> * more IO, we should wait for the application to
> * reap events and wake us up.
> */
> - if (inflight ||
> + if (!list_empty(&ctx->poll_list) ||
> (!time_after(jiffies, timeout) && ret != -EBUSY &&
> !percpu_ref_is_dying(&ctx->refs))) {
> cond_resched();
> @@ -5089,6 +5074,24 @@ static int io_sq_thread(void *data)
> cur_mm = NULL;
> }
>
> + /*
> + * While doing polled IO, before going to sleep, we need
> + * to check if there are new reqs added to poll_list, it
> + * is because reqs may have been punted to io worker and
> + * will be added to poll_list later, hence check the
> + * poll_list again, meanwhile we need to hold uring_lock
> + * to do this check, otherwise we may lose wakeup event
> + * in io_iopoll_req_issued().
> + */
> + if (needs_uring_lock) {
> + mutex_lock(&ctx->uring_lock);
> + if (!list_empty(&ctx->poll_list)) {
> + mutex_unlock(&ctx->uring_lock);
> + cond_resched();
> + continue;
> + }
> + }
> +
> prepare_to_wait(&ctx->sqo_wait, &wait,
> TASK_INTERRUPTIBLE);
>
> @@ -5101,16 +5104,22 @@ static int io_sq_thread(void *data)
> if (!to_submit || ret == -EBUSY) {
> if (kthread_should_park()) {
> finish_wait(&ctx->sqo_wait, &wait);
> + if (needs_uring_lock)
> + mutex_unlock(&ctx->uring_lock);
> break;
> }
> if (signal_pending(current))
> flush_signals(current);
> + if (needs_uring_lock)
> + mutex_unlock(&ctx->uring_lock);
> schedule();
> finish_wait(&ctx->sqo_wait, &wait);
>
> ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
> continue;
> }
> + if (needs_uring_lock)
> + mutex_unlock(&ctx->uring_lock);
> finish_wait(&ctx->sqo_wait, &wait);
>
> ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
> @@ -5119,8 +5128,7 @@ static int io_sq_thread(void *data)
> mutex_lock(&ctx->uring_lock);
> ret = io_submit_sqes(ctx, to_submit, NULL, -1, &cur_mm, true);
> mutex_unlock(&ctx->uring_lock);
> - if (ret > 0)
> - inflight += ret;
> + timeout = jiffies + ctx->sq_thread_idle;
> }
>
> set_fs(old_fs);
>
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