Reading the SQ tail needs to come after setting IORING_SQ_NEED_WAKEUP in
flags; there is no cheap barrier for ordering a store before a load, a
full memory barrier is required.
Userspace needs a full memory barrier between updating SQ tail and
checking for the IORING_SQ_NEED_WAKEUP too.
Signed-off-by: Stefan Bühler <source@xxxxxxxxxxxx>
---
fs/io_uring.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 96863e4780b7..912b0b304d90 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1865,7 +1865,8 @@ static int io_sq_thread(void *data)
/* Tell userspace we may need a wakeup call */
ctx->sq_ring->flags |= IORING_SQ_NEED_WAKEUP;
- smp_wmb();
+ /* make sure to read SQ tail after writing flags */
+ smp_mb();
if (!io_get_sqring(ctx, &sqes[0])) {
if (kthread_should_stop()) {
--
2.20.1
