Re: [PATCH v3] io_uring: fix poll_list race for SETUP_IOPOLL|SETUP_SQPOLL

Jens Axboe
2020-02-24T20:10:35Z

On 2/24/20 12:03 AM, Xiaoguang Wang wrote:
> 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.

I'm still not a huge fan of this solution. A few comments below:

> + if (!list_empty(&ctx->poll_list))
> + io_iopoll_getevents(ctx, &nr_events, 0);
> + if (list_empty(&ctx->poll_list))
> timeout = jiffies + ctx->sq_thread_idle;

Just use an else?

> + /*
> + * 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;
> + }
> + }

Can't we just put this below the prepare_to_wait? I'm not convinced
this is closing the gaps, there should be no need to hold the uring
lock over this long stretch.

Modified version of yours below

diff --git a/fs/io_uring.c b/fs/io_uring.c
index d961945cb332..ffd9bfa84d86 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1821,6 +1821,10 @@ 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)
@@ -5086,9 +5090,8 @@ 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;

complete(&ctx->completions[1]);

@@ -5096,39 +5099,19 @@ static int io_sq_thread(void *data)
set_fs(USER_DS);
old_cred = override_creds(ctx->creds);

- ret = timeout = inflight = 0;
+ 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_getevents(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_getevents(ctx, &nr_events, 0);
+ else
timeout = jiffies + ctx->sq_thread_idle;
+ mutex_unlock(&ctx->uring_lock);
}

to_submit = io_sqring_entries(ctx);
@@ -5157,7 +5140,7 @@ 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();
@@ -5167,6 +5150,19 @@ static int io_sq_thread(void *data)
prepare_to_wait(&ctx->sqo_wait, &wait,
TASK_INTERRUPTIBLE);

+ /*
+ * 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.
+ */
+ if ((ctx->flags & IORING_SETUP_IOPOLL) &&
+ !list_empty_careful(&ctx->poll_list)) {
+ finish_wait(&ctx->sqo_wait, &wait);
+ continue;
+ }
+
/* Tell userspace we may need a wakeup call */
ctx->rings->sq_flags |= IORING_SQ_NEED_WAKEUP;
/* make sure to read SQ tail after writing flags */
@@ -5194,8 +5190,7 @@ 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);

--
Jens Axboe