io_uring: fix overflow resched cqe reordering

[ Upstream commit a7d755ed9ce9738af3db602eb29d32774a180bc7 ]

Leaving the CQ critical section in the middle of a overflow flushing
can cause cqe reordering since the cache cq pointers are reset and any
new cqe emitters that might get called in between are not going to be
forced into io_cqe_cache_refill().

Fixes: eac2ca2d68 ("io_uring: check if we need to reschedule during overflow flush")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/90ba817f1a458f091f355f407de1c911d2b93bbf.1747483784.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

io_uring/io_uring.c

@@ -630,6 +630,7 @@ static void __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool dying)
 		 * to care for a non-real case.
 		 */
 		if (need_resched()) {
+			ctx->cqe_sentinel = ctx->cqe_cached;
 			io_cq_unlock_post(ctx);
 			mutex_unlock(&ctx->uring_lock);
 			cond_resched();