[PATCH] io_uring: use cond_resched() in io_ring_ctx_wait_and_kill()

[PATCH] io_uring: use cond_resched() in io_ring_ctx_wait_and_kill()

[Xiaoguang Wang]

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.

Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
---
 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 0c5d25ae51b9..2bc4dad1aacd 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -7340,7 +7340,7 @@ 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);
-- 
2.17.2

Re: [PATCH] io_uring: use cond_resched() in io_ring_ctx_wait_and_kill()

[Jens Axboe]

On 4/30/20 6:52 PM, Xiaoguang Wang wrote:
> 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.

Thanks, this looks good, I believe it should also fix the syzbot reported
issue so I added the tag for that.

-- 
Jens Axboe