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

Pavel Begunkov
2021-01-26T16:01:16Z

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