io_uring: use cond_resched() in io_ring_ctx_wait_and_kill()

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>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 3da2a02..5ca2da6 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -7343,7 +7343,7 @@ 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);