IORING_SETUP_SINGLE_ISSUER doesn't currently enable any optimizations,
but it will soon be used to avoid taking io_ring_ctx's uring_lock when
submitting from the single issuer task. If the IORING_SETUP_SQPOLL flag
is set, the SQ thread is the sole task issuing SQEs. However, other
tasks may make io_uring_register() syscalls, which must be synchronized
with SQE submission. So it wouldn't be safe to skip the uring_lock
around the SQ thread's submission even if IORING_SETUP_SINGLE_ISSUER is
set. Therefore, clear IORING_SETUP_SINGLE_ISSUER from the io_ring_ctx
flags if IORING_SETUP_SQPOLL is set.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
---
 io_uring/io_uring.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 1e58fc1d5667..05a1ac457581 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -3473,10 +3473,19 @@ static int io_uring_sanitise_params(struct io_uring_params *p)
 	 */
 	if ((flags & (IORING_SETUP_SQE128|IORING_SETUP_SQE_MIXED)) ==
 	    (IORING_SETUP_SQE128|IORING_SETUP_SQE_MIXED))
 		return -EINVAL;
 
+	/*
+	 * If IORING_SETUP_SQPOLL is set, only the SQ thread issues SQEs,
+	 * but other threads may call io_uring_register() concurrently.
+	 * We still need uring_lock to synchronize these io_ring_ctx accesses,
+	 * so disable the single issuer optimizations.
+	 */
+	if (flags & IORING_SETUP_SQPOLL)
+		p->flags &= ~IORING_SETUP_SINGLE_ISSUER;
+
 	return 0;
 }
 
 static int io_uring_fill_params(struct io_uring_params *p)
 {
-- 
2.45.2