[PATCH] iouring: Fix min_timeout behaviour

[Christian A. Ehrhardt]

The wakeup condition if a min timeout is present and has
expired is that at least _one_ CQE was posted. Thus set
the cq_tail target to ->cq_min_tail + 1. Without this
commit a spurious wakeup can result in a premature wakeup
because io_should_wake() will return true even if _no_ CQE
was posted at all.

Tested by running the liburing testsuite with no regressions.

Additionally, tested by turning all calls to schedule() in
io_uring/wait.c into calls to schedule_timeout(1) to force
the spurious wakeups. With these spurious wakeups the
min-timeout.t test fails before and passes after this commit.

Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Tip ten Brink <tip@xxxxxxxxxxxxxxxxx>
Fixes: e15cb2200b93 ("io_uring: fix min_wait wakeups for SQPOLL")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Christian A. Ehrhardt <lk@xxxxxxx>
---
 io_uring/wait.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/io_uring/wait.c b/io_uring/wait.c
index ec01e78a216d..d005ea17b35f 100644
--- a/io_uring/wait.c
+++ b/io_uring/wait.c
@@ -103,7 +103,7 @@ static enum hrtimer_restart io_cqring_min_timer_wakeup(struct hrtimer *timer)
 	}
 
 	/* any generated CQE posted past this time should wake us up */
-	iowq->cq_tail = iowq->cq_min_tail;
+	iowq->cq_tail = iowq->cq_min_tail + 1;
 
 	hrtimer_update_function(&iowq->t, io_cqring_timer_wakeup);
 	hrtimer_set_expires(timer, iowq->timeout);
-- 
2.43.0