On 8/7/20 3:50 PM, Jens Axboe wrote:
> On 8/7/20 12:00 PM, Jann Horn wrote:
>> On Fri, Aug 7, 2020 at 6:56 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>
>>> An earlier commit:
>>>
>>> b7db41c9e03b ("io_uring: fix regression with always ignoring signals in io_cqring_wait()")
>>>
>>> ensured that we didn't get stuck waiting for eventfd reads when it's
>>> registered with the io_uring ring for event notification, but we still
>>> have a gap where the task can be waiting on other events in the kernel
>>> and need a bigger nudge to make forward progress.
>>>
>>> Ensure that we use signaled notifications for a task that isn't currently
>>> running, to be certain the work is seen and processed immediately.
>>>
>>> Cc: stable@vger.kernel.org # v5.7+
>>> Reported-by: Josef <josef.grieb@gmail.com>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>
>>> ---
>>>
>>> This isn't perfect, as it'll use TWA_SIGNAL even for cases where we
>>> don't absolutely need it (like task waiting for completions in
>>> io_cqring_wait()), but we don't have a good way to tell right now. We
>>> can probably improve on this in the future, for now I think this is the
>>> best solution.
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index e9b27cdaa735..b4300a61f231 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -1720,7 +1720,7 @@ static int io_req_task_work_add(struct io_kiocb *req, struct callback_head *cb)
>>>          */
>>>         if (ctx->flags & IORING_SETUP_SQPOLL)
>>>                 notify = 0;
>>> -       else if (ctx->cq_ev_fd)
>>> +       else if (ctx->cq_ev_fd || (tsk->state != TASK_RUNNING))
>>>                 notify = TWA_SIGNAL;
>>>
>>>         ret = task_work_add(tsk, cb, notify);
>>
>> I don't get it. Apart from still not understanding the big picture:
>>
>> What guarantees that the lockless read of tsk->state is in any way
>> related to the state of the remote process by the time we reach
>> task_work_add()? And why do we not need to signal in TASK_RUNNING
>> state (e.g. directly before the remote process switches to
>> TASK_INTERRUPTIBLE or something like that)?
> 
> Yeah it doesn't, the patch doesn't cover the racy case. As far as I can
> tell, we've got two ways to do it:
> 
> 1) We split the task_work_add() into two parts, one adding the work and
>    one doing the signaling. Then we could do:
> 
> int notify = TWA_RESUME;
> 
> __task_work_add(tsk, cb);
> 
> if (ctx->flags & IORING_SETUP_SQPOLL)
> 	notify = 0;
> else if (ctx->cq_ev_fd || (tsk->state != TASK_RUNNING))
> 	notify = TWA_SIGNAL;
> 
> __task_work_signal(tsk, notify);

Something like the attached - totally untested so far, but it implements
that idea.

-- 
Jens Axboe