Don't let io_uring_wait_cqes() submit IO
It's a wait interface, if we let it it also touch the SQ side of things,
then it's not possible to use sanely in a situation where someone has
a separate thread for completions.
Note that this may require application changes, if they are currently
relying on io_uring_wait_cqes() on also submitting IO. The risk should
be minimal however, since it is a wait-for-completions interface.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/src/queue.c b/src/queue.c
index 74f023c..7dec0c0 100644
--- a/src/queue.c
+++ b/src/queue.c
@@ -101,16 +101,20 @@
* this function must never set sqe->user_data to LIBURING_UDATA_TIMEOUT!
*
* Note that the application need not call io_uring_submit() before calling
- * this function, as we will do that on its behalf.
+ * this function, as we will do that on its behalf. From this it also follows
+ * that this function isn't safe to use for applications that split SQ and CQ
+ * handling between two threads and expect that to work without synchronization,
+ * as this function manipulates both the SQ and CQ side.
*/
int io_uring_wait_cqes(struct io_uring *ring, struct io_uring_cqe **cqe_ptr,
unsigned wait_nr, struct __kernel_timespec *ts,
sigset_t *sigmask)
{
- int ret;
+ unsigned to_submit = 0;
if (wait_nr && ts) {
struct io_uring_sqe *sqe;
+ int ret;
/*
* If the SQ ring is full, we may need to submit IO first
@@ -124,16 +128,15 @@
}
io_uring_prep_timeout(sqe, ts, wait_nr, 0);
sqe->user_data = LIBURING_UDATA_TIMEOUT;
+ to_submit = __io_uring_flush_sq(ring);
}
- ret = __io_uring_flush_sq(ring);
-
- return __io_uring_get_cqe(ring, cqe_ptr, ret, wait_nr, sigmask);
+ return __io_uring_get_cqe(ring, cqe_ptr, to_submit, wait_nr, sigmask);
}
/*
- * See io_uring_wait_cqes_timeout() - this function is the same, it just
- * always uses '1' as the wait_nr.
+ * See io_uring_wait_cqes() - this function is the same, it just always uses
+ * '1' as the wait_nr.
*/
int io_uring_wait_cqe_timeout(struct io_uring *ring,
struct io_uring_cqe **cqe_ptr,