[PATCH liburing] Fix hang in in io_uring_get_cqe() with iopoll

[PATCH liburing] Fix hang in in io_uring_get_cqe() with iopoll

[Pavel Begunkov]

Because of need_resched() check, io_uring_enter() -> io_iopoll_check()
can return 0 even if @min_complete wasn't satisfied. If that's the
case, __io_uring_get_cqe() sets submit=0 and wait_nr=0, disabling
setting IORING_ENTER_GETEVENTS as well. So, it goes crazy calling
io_uring_enter() in a loop, not actually submitting nor polling.

Set @wait_nr based on actual number of CQEs ready.
BTW, atomic_load_acquire() in io_uring_cq_ready() can be replaced
with a relaxed one for this particular place.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 src/queue.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/queue.c b/src/queue.c
index 14a0777..638d0ac 100644
--- a/src/queue.c
+++ b/src/queue.c
@@ -32,6 +32,14 @@ static inline bool sq_ring_needs_enter(struct io_uring *ring,
 	return false;
 }
 
+static inline unsigned int io_adjut_wait_nr(struct io_uring *ring,
+					    unsigned int to_wait)
+{
+	unsigned int ready = io_uring_cq_ready(ring);
+
+	return (to_wait <= ready) ? 0 : (to_wait - ready);
+}
+
 int __io_uring_get_cqe(struct io_uring *ring, struct io_uring_cqe **cqe_ptr,
 		       unsigned submit, unsigned wait_nr, sigset_t *sigmask)
 {
@@ -60,7 +68,8 @@ int __io_uring_get_cqe(struct io_uring *ring, struct io_uring_cqe **cqe_ptr,
 			err = -errno;
 		} else if (ret == (int)submit) {
 			submit = 0;
-			wait_nr = 0;
+			if (to_wait)
+				wait_nr = io_adjut_wait_nr(ring, to_wait);
 		} else {
 			submit -= ret;
 		}
-- 
2.24.0

Re: [PATCH liburing] Fix hang in in io_uring_get_cqe() with iopoll

[Jens Axboe]

On 6/21/20 4:30 AM, Pavel Begunkov wrote:
> Because of need_resched() check, io_uring_enter() -> io_iopoll_check()
> can return 0 even if @min_complete wasn't satisfied. If that's the
> case, __io_uring_get_cqe() sets submit=0 and wait_nr=0, disabling
> setting IORING_ENTER_GETEVENTS as well. So, it goes crazy calling
> io_uring_enter() in a loop, not actually submitting nor polling.
> 
> Set @wait_nr based on actual number of CQEs ready.
> BTW, atomic_load_acquire() in io_uring_cq_ready() can be replaced
> with a relaxed one for this particular place.

Can you preface this with an addition of __io_uring_cqe_ready() that
doesn't include the load acquire?

Also, s/io_adjut_wait_nr/io_adjust_wait_nr for the patch.

-- 
Jens Axboe

Re: [PATCH liburing] Fix hang in in io_uring_get_cqe() with iopoll

[Pavel Begunkov]

On 21/06/2020 18:23, Jens Axboe wrote:
> On 6/21/20 4:30 AM, Pavel Begunkov wrote:
>> Because of need_resched() check, io_uring_enter() -> io_iopoll_check()
>> can return 0 even if @min_complete wasn't satisfied. If that's the
>> case, __io_uring_get_cqe() sets submit=0 and wait_nr=0, disabling
>> setting IORING_ENTER_GETEVENTS as well. So, it goes crazy calling
>> io_uring_enter() in a loop, not actually submitting nor polling.
>>
>> Set @wait_nr based on actual number of CQEs ready.
>> BTW, atomic_load_acquire() in io_uring_cq_ready() can be replaced
>> with a relaxed one for this particular place.
> 
> Can you preface this with an addition of __io_uring_cqe_ready() that
> doesn't include the load acquire?

Sure

> Also, s/io_adjut_wait_nr/io_adjust_wait_nr for the patch.
Ouch, a typo


-- 
Pavel Begunkov