Re: [PATCH 2/2] io_uring: use TWA_SIGNAL for task_work if the task isn't running

[Jens Axboe <axboe@kernel.dk>]

On 8/10/20 5:42 AM, peterz@infradead.org wrote:
> On Sat, Aug 08, 2020 at 12:34:39PM -0600, Jens Axboe 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>
>> ---
>>  fs/io_uring.c | 22 ++++++++++++++--------
>>  1 file changed, 14 insertions(+), 8 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index e9b27cdaa735..443eecdfeda9 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -1712,21 +1712,27 @@ static int io_req_task_work_add(struct io_kiocb *req, struct callback_head *cb)
>>  	struct io_ring_ctx *ctx = req->ctx;
>>  	int ret, notify = TWA_RESUME;
>>  
>> +	ret = __task_work_add(tsk, cb);
>> +	if (unlikely(ret))
>> +		return ret;
>> +
>>  	/*
>>  	 * SQPOLL kernel thread doesn't need notification, just a wakeup.
>> -	 * If we're not using an eventfd, then TWA_RESUME is always fine,
>> -	 * as we won't have dependencies between request completions for
>> -	 * other kernel wait conditions.
>> +	 * For any other work, use signaled wakeups if the task isn't
>> +	 * running to avoid dependencies between tasks or threads. If
>> +	 * the issuing task is currently waiting in the kernel on a thread,
>> +	 * and same thread is waiting for a completion event, then we need
>> +	 * to ensure that the issuing task processes task_work. TWA_SIGNAL
>> +	 * is needed for that.
>>  	 */
>>  	if (ctx->flags & IORING_SETUP_SQPOLL)
>>  		notify = 0;
>> -	else if (ctx->cq_ev_fd)
>> +	else if (READ_ONCE(tsk->state) != TASK_RUNNING)
>>  		notify = TWA_SIGNAL;
>>  
>> -	ret = task_work_add(tsk, cb, notify);
>> -	if (!ret)
>> -		wake_up_process(tsk);
>> -	return ret;
>> +	__task_work_notify(tsk, notify);
>> +	wake_up_process(tsk);
>> +	return 0;
>>  }
> 
> Wait.. so the only change here is that you look at tsk->state, _after_
> doing __task_work_add(), but nothing, not the Changelog nor the comment
> explains this.
> 
> So you're relying on __task_work_add() being an smp_mb() vs the add, and
> you order this against the smp_mb() in set_current_state() ?
> 
> This really needs spelling out.

I'll update the changelog, it suffers a bit from having been reused from
the earlier versions. Thanks for checking!

-- 
Jens Axboe