On 06/06/2020 18:12, Xiaoguang Wang wrote:
> While testing io_uring in our internal kernel, note it's not upstream
> kernel, we see below panic:
> [ 872.498723] x29: ffff00002d553cf0 x28: 0000000000000000
> [ 872.508973] x27: ffff807ef691a0e0 x26: 0000000000000000
> [ 872.519116] x25: 0000000000000000 x24: ffff0000090a7980
> [ 872.529184] x23: ffff000009272060 x22: 0000000100022b11
> [ 872.539144] x21: 0000000046aa5668 x20: ffff80bee8562b18
> [ 872.549000] x19: ffff80bee8562080 x18: 0000000000000000
> [ 872.558876] x17: 0000000000000000 x16: 0000000000000000
> [ 872.568976] x15: 0000000000000000 x14: 0000000000000000
> [ 872.578762] x13: 0000000000000000 x12: 0000000000000000
> [ 872.588474] x11: 0000000000000000 x10: 0000000000000c40
> [ 872.598324] x9 : ffff000008100c00 x8 : 000000007ffff000
> [ 872.608014] x7 : ffff80bee8562080 x6 : ffff80beea862d30
> [ 872.617709] x5 : 0000000000000000 x4 : ffff80beea862d48
> [ 872.627399] x3 : ffff80bee8562b18 x2 : 0000000000000000
> [ 872.637044] x1 : ffff0000090a7000 x0 : 0000000000208040
> [ 872.646575] Call trace:
> [ 872.653139] task_numa_work+0x4c/0x310
> [ 872.660916] task_work_run+0xb0/0xe0
> [ 872.668400] io_sq_thread+0x164/0x388
> [ 872.675829] kthread+0x108/0x138
>
> The reason is that once io_sq_thread has a valid mm, schedule subsystem
> may call task_tick_numa() adding a task_numa_work() callback, which will
> visit mm, then above panic will happen.
>
> To fix this bug, only call task_work_run() before dropping mm.
So, the problem is that poll/async paths re-issue requests with
__io_queue_sqe(), which doesn't care about current->mm, and which
can be NULL for io_sq_thread(). Right?
>
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx>
> ---
> fs/io_uring.c | 15 ++++++++-------
> 1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 6391a00ff8b7..32381984b2a6 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -6134,6 +6134,13 @@ static int io_sq_thread(void *data)
> * to enter the kernel to reap and flush events.
> */
> if (!to_submit || ret == -EBUSY) {
> + /*
> + * Current task context may already have valid mm, that
> + * means some works that visit mm may have been queued,
> + * so we must execute the works before dropping mm.
> + */
> + if (current->task_works)
> + task_work_run();
Even though you're not dropping mm, the thread might not have it in the first
place. see how it's done in io_init_req(). How about setting mm either lazily
in io_poll_task_func()/io_async_task_func(), or before task_work_run() in
io_sq_thread().
> /*
> * Drop cur_mm before scheduling, we can't hold it for
> * long periods (or over schedule()). Do this before
> @@ -6152,8 +6159,6 @@ static int io_sq_thread(void *data)
> if (!list_empty(&ctx->poll_list) ||
> (!time_after(jiffies, timeout) && ret != -EBUSY &&
> !percpu_ref_is_dying(&ctx->refs))) {
> - if (current->task_works)
> - task_work_run();
> cond_resched();
> continue;
> }
> @@ -6185,11 +6190,7 @@ static int io_sq_thread(void *data)
> finish_wait(&ctx->sqo_wait, &wait);
> break;
> }
> - if (current->task_works) {
> - task_work_run();
> - finish_wait(&ctx->sqo_wait, &wait);
> - continue;
> - }
> +
> if (signal_pending(current))
> flush_signals(current);
> schedule();
>
--
Pavel Begunkov
