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.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

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);