io_uring: mitigate unlikely iopoll lag

We have requests like IORING_OP_FILES_UPDATE that don't go through
->iopoll_list but get completed in place under ->uring_lock, and so
after dropping the lock io_iopoll_check() should expect that some CQEs
might have get completed in a meanwhile.

Currently such events won't be accounted in @nr_events, and the loop
will continue to poll even if there is enough of CQEs. It shouldn't be a
problem as it's not likely to happen and so, but not nice either. Just
return earlier in this case, it should be enough.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/66ef932cc66a34e3771bbae04b2953a8058e9d05.1625747741.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 8f2a66903f5ad7bce35e676a08885c3463231e97..7167c61c6d1b8295ebd2cc3d61e4c75ab70525b6 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2356,11 +2356,15 @@ static int io_iopoll_check(struct io_ring_ctx *ctx, long min)
                 * very same mutex.
                 */
                if (list_empty(&ctx->iopoll_list)) {
+                       u32 tail = ctx->cached_cq_tail;
+
                        mutex_unlock(&ctx->uring_lock);
                        io_run_task_work();
                        mutex_lock(&ctx->uring_lock);
 
-                       if (list_empty(&ctx->iopoll_list))
+                       /* some requests don't go through iopoll_list */
+                       if (tail != ctx->cached_cq_tail ||
+                           list_empty(&ctx->iopoll_list))
                                break;
                }
                ret = io_do_iopoll(ctx, &nr_events, min);