On 8/11/20 9:23 AM, Oleg Nesterov wrote:
> On 08/11, Jens Axboe wrote:
>>
>> --- a/kernel/task_work.c
>> +++ b/kernel/task_work.c
>> @@ -42,7 +42,8 @@ task_work_add(struct task_struct *task, struct
>> callback_head *work, int notify)
>> set_notify_resume(task);
>> break;
>> case TWA_SIGNAL:
>> - if (lock_task_sighand(task, &flags)) {
>> + if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
>> + lock_task_sighand(task, &flags)) {
>
> Aaaaah, sorry Jens, now I think this is racy. So I am glad I didn't add
> this optimization into the initial version ;)
>
> It is possible that JOBCTL_TASK_WORK is set but ->task_works == NULL. Say,
> task_work_add(TWA_SIGNAL) + task_work_cancel(), or the target task can call
> task_work_run() before it enters get_signal().
>
> And in this case another task_work_add(tsk, TWA_SIGNAL) can actually race
> with get_signal() which does
>
> current->jobctl &= ~JOBCTL_TASK_WORK;
> if (unlikely(current->task_works)) {
> spin_unlock_irq(&sighand->siglock);
> task_work_run();
>
> nothing guarantees that get_signal() sees ->task_works != NULL. Probably
> this is what Jann meant.
>
> We can probably add a barrier into get_signal() but I didn't sleep today,
> I'll try to think tomorrow.
Appreciate you looking into this! Would be pretty critical for me to get
this working.
--
Jens Axboe
