From 8f487ef2cbb2d4f6ca8c113d70da63baaf68c91a Mon Sep 17 00:00:00 2001
From: Pavel Begunkov <asml.silence@gmail.com>
Date: Thu, 8 Jul 2021 13:37:06 +0100
Subject: 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>
---
 fs/io_uring.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

(limited to 'fs/io_uring.c')

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 8f2a66903f5a..7167c61c6d1b 100644
--- 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);
-- 
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