[PATCH 5.6 067/194] io_uring: use cond_resched() in io_ring_ctx_wait_and_kill()

Greg Kroah-Hartman
2020-05-18T18:02:34Z

From: Xiaoguang Wang <xiaoguang.wang@linux.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@syzkaller.appspotmail.com
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.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