io_uring: check if we need to reschedule during overflow flush

In terms of normal application usage, this list will always be empty.
And if an application does overflow a bit, it'll have a few entries.
However, nothing obviously prevents syzbot from running a test case
that generates a ton of overflow entries, and then flushing them can
take quite a while.

Check for needing to reschedule while flushing, and drop our locks and
do so if necessary. There's no state to maintain here as overflows
always prune from head-of-list, hence it's fine to drop and reacquire
the locks at the end of the loop.

Link: https://lore.kernel.org/io-uring/66ed061d.050a0220.29194.0053.GAE@google.com/
Reported-by: syzbot+5fca234bd7eb378ff78e@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -624,6 +624,21 @@ static void __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool dying)
 		}
 		list_del(&ocqe->list);
 		kfree(ocqe);
+
+		/*
+		 * For silly syzbot cases that deliberately overflow by huge
+		 * amounts, check if we need to resched and drop and
+		 * reacquire the locks if so. Nothing real would ever hit this.
+		 * Ideally we'd have a non-posting unlock for this, but hard
+		 * to care for a non-real case.
+		 */
+		if (need_resched()) {
+			io_cq_unlock_post(ctx);
+			mutex_unlock(&ctx->uring_lock);
+			cond_resched();
+			mutex_lock(&ctx->uring_lock);
+			io_cq_lock(ctx);
+		}
 	}
 
 	if (list_empty(&ctx->cq_overflow_list)) {