[PATCH v2 2/2] Fix hang in io_uring_get_cqe() with iopoll

[Pavel Begunkov <asml.silence@gmail.com>]

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. It doesn't manifest
extra CQEs if any, thus implements __io_uring_cq_ready() with relaxed
semantics.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 src/queue.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/queue.c b/src/queue.c
index 14a0777..d824cfd 100644
--- a/src/queue.c
+++ b/src/queue.c
@@ -32,6 +32,19 @@ static inline bool sq_ring_needs_enter(struct io_uring *ring,
 	return false;
 }
 
+static inline unsigned int __io_uring_cq_ready(struct io_uring *ring)
+{
+	return io_uring_smp_load_relaxed(ring->cq.ktail) - *ring->cq.khead;
+}
+
+static inline unsigned int io_adjust_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 +73,8 @@ 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_adjust_wait_nr(ring, to_wait);
 		} else {
 			submit -= ret;
 		}
-- 
2.24.0