Re: [PATCH 2/2] io_uring: fix cancellation taking mutex while TASK_UNINTERRUPTIBLE

[Pavel Begunkov <asml.silence@gmail.com>]

On 26/01/2021 15:28, Pavel Begunkov wrote:
> do not call blocking ops when !TASK_RUNNING; state=2 set at
> 	[<00000000ced9dbfc>] prepare_to_wait+0x1f4/0x3b0
> 	kernel/sched/wait.c:262
> WARNING: CPU: 1 PID: 19888 at kernel/sched/core.c:7853
> 	__might_sleep+0xed/0x100 kernel/sched/core.c:7848
> RIP: 0010:__might_sleep+0xed/0x100 kernel/sched/core.c:7848
> Call Trace:
>  __mutex_lock_common+0xc4/0x2ef0 kernel/locking/mutex.c:935
>  __mutex_lock kernel/locking/mutex.c:1103 [inline]
>  mutex_lock_nested+0x1a/0x20 kernel/locking/mutex.c:1118
>  io_wq_submit_work+0x39a/0x720 fs/io_uring.c:6411
>  io_run_cancel fs/io-wq.c:856 [inline]
>  io_wqe_cancel_pending_work fs/io-wq.c:990 [inline]
>  io_wq_cancel_cb+0x614/0xcb0 fs/io-wq.c:1027
>  io_uring_cancel_files fs/io_uring.c:8874 [inline]
>  io_uring_cancel_task_requests fs/io_uring.c:8952 [inline]
>  __io_uring_files_cancel+0x115d/0x19e0 fs/io_uring.c:9038
>  io_uring_files_cancel include/linux/io_uring.h:51 [inline]
>  do_exit+0x2e6/0x2490 kernel/exit.c:780
>  do_group_exit+0x168/0x2d0 kernel/exit.c:922
>  get_signal+0x16b5/0x2030 kernel/signal.c:2770
>  arch_do_signal_or_restart+0x8e/0x6a0 arch/x86/kernel/signal.c:811
>  handle_signal_work kernel/entry/common.c:147 [inline]
>  exit_to_user_mode_loop kernel/entry/common.c:171 [inline]
>  exit_to_user_mode_prepare+0xac/0x1e0 kernel/entry/common.c:201
>  __syscall_exit_to_user_mode_work kernel/entry/common.c:291 [inline]
>  syscall_exit_to_user_mode+0x48/0x190 kernel/entry/common.c:302
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Rewrite io_uring_cancel_files() to mimic __io_uring_task_cancel()'s
> counting scheme, so it does all the heavy work before setting
> TASK_UNINTERRUPTIBLE.
> 
> Cc: stable@vger.kernel.org # 5.9+
> Reported-by: syzbot+f655445043a26a7cfab8@syzkaller.appspotmail.com
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  fs/io_uring.c | 41 ++++++++++++++++++++++++-----------------
>  1 file changed, 24 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 09aada153a71..f3f2b37e7021 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -8873,30 +8873,33 @@ static void io_cancel_defer_files(struct io_ring_ctx *ctx,
>  	}
>  }
>  
> +static int io_uring_count_inflight(struct io_ring_ctx *ctx,
> +				   struct task_struct *task,
> +				   struct files_struct *files)
> +{
> +	struct io_kiocb *req;
> +	int cnt = 0;
> +
> +	spin_lock_irq(&ctx->inflight_lock);
> +	list_for_each_entry(req, &ctx->inflight_list, inflight_entry) {
> +		if (!io_match_task(req, task, files))

This condition should be inversed. Jens, please drop it

p.s. I wonder how tests didn't catch that

> +			cnt++;
> +	}
> +	spin_unlock_irq(&ctx->inflight_lock);
> +	return cnt;
> +}
> +
>  static void io_uring_cancel_files(struct io_ring_ctx *ctx,
>  				  struct task_struct *task,
>  				  struct files_struct *files)
>  {
>  	while (!list_empty_careful(&ctx->inflight_list)) {
>  		struct io_task_cancel cancel = { .task = task, .files = files };
> -		struct io_kiocb *req;
>  		DEFINE_WAIT(wait);
> -		bool found = false;
> +		int inflight;
>  
> -		spin_lock_irq(&ctx->inflight_lock);
> -		list_for_each_entry(req, &ctx->inflight_list, inflight_entry) {
> -			if (!io_match_task(req, task, files))
> -				continue;
> -			found = true;
> -			break;
> -		}
> -		if (found)
> -			prepare_to_wait(&task->io_uring->wait, &wait,
> -					TASK_UNINTERRUPTIBLE);
> -		spin_unlock_irq(&ctx->inflight_lock);
> -
> -		/* We need to keep going until we don't find a matching req */
> -		if (!found)
> +		inflight = io_uring_count_inflight(ctx, task, files);
> +		if (!inflight)
>  			break;
>  
>  		io_wq_cancel_cb(ctx->io_wq, io_cancel_task_cb, &cancel, true);
> @@ -8905,7 +8908,11 @@ static void io_uring_cancel_files(struct io_ring_ctx *ctx,
>  		io_cqring_overflow_flush(ctx, true, task, files);
>  		/* cancellations _may_ trigger task work */
>  		io_run_task_work();
> -		schedule();
> +
> +		prepare_to_wait(&task->io_uring->wait, &wait,
> +				TASK_UNINTERRUPTIBLE);
> +		if (inflight == io_uring_count_inflight(ctx, task, files))
> +			schedule();
>  		finish_wait(&task->io_uring->wait, &wait);
>  	}
>  }
> 

-- 
Pavel Begunkov