linux-kernel - [PATCH 5.6 067/194] io_uring: use cond_resched() in io_ring_ctx_wait_and

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Message-Id: <20200518173537.317015147@linuxfoundation.org>
Date:   Mon, 18 May 2020 19:35:57 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>,
        Jens Axboe <axboe@...nel.dk>, Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.6 067/194] io_uring: use cond_resched() in io_ring_ctx_wait_and_kill()

From: Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>

[ Upstream commit 3fd44c86711f71156b586c22b0495c58f69358bb ]

While working on to make io_uring sqpoll mode support syscalls that need
struct files_struct, I got cpu soft lockup in io_ring_ctx_wait_and_kill(),

    while (ctx->sqo_thread && !wq_has_sleeper(&ctx->sqo_wait))
        cpu_relax();

above loop never has an chance to exit, it's because preempt isn't enabled
in the kernel, and the context calling io_ring_ctx_wait_and_kill() and
io_sq_thread() run in the same cpu, if io_sq_thread calls a cond_resched()
yield cpu and another context enters above loop, then io_sq_thread() will
always in runqueue and never exit.

Use cond_resched() can fix this issue.

 Reported-by: syzbot+66243bb7126c410cefe6@...kaller.appspotmail.com
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 fs/io_uring.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 9690c845a3e4b..01f71b9efb88f 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6451,7 +6451,7 @@ static void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
 	 * it could cause shutdown to hang.
 	 */
 	while (ctx->sqo_thread && !wq_has_sleeper(&ctx->sqo_wait))
-		cpu_relax();
+		cond_resched();
 
 	io_kill_timeouts(ctx);
 	io_poll_remove_all(ctx);
-- 
2.20.1