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SubjectRe: [PATCH 3/3] task_work: use TIF_TASKWORK if available
Heh. To be honest I don't really like 1-2 ;)

Unfortunately, I do not see a better approach right now. Let me think
until Monday, it is not that I think I will find a better solution, but
I'd like to try anyway.

Let me comment 3/3 for now.

On 10/01, Jens Axboe wrote:
>
> +static void task_work_signal(struct task_struct *task)
> +{
> +#ifndef TIF_TASKWORK
> + unsigned long flags;
> +
> + /*
> + * Only grab the sighand lock if we don't already have some
> + * task_work pending. This pairs with the smp_store_mb()
> + * in get_signal(), see comment there.
> + */
> + if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
> + lock_task_sighand(task, &flags)) {
> + task->jobctl |= JOBCTL_TASK_WORK;
> + signal_wake_up(task, 0);
> + unlock_task_sighand(task, &flags);
> + }
> +#else
> + set_tsk_thread_flag(task, TIF_TASKWORK);
> + set_notify_resume(task);
> +#endif

Again, I can't understand. task_work_signal(task) should set TIF_TASKWORK
to make signal_pending() = T _and_ wake/kick the target up, just like
signal_wake_up() does. Why do we set TIF_NOTIFY_RESUME ?

So I think that if we are going to add TIF_TASKWORK we should generalize
this logic and turn it into TIF_NOTIFY_SIGNAL. Similar to TIF_NOTIFY_RESUME
but implies signal_pending().

IOW, something like

void set_notify_signal(task)
{
if (!test_and_set_tsk_thread_flag(task, TIF_NOTIFY_SIGNAL)) {
if (!wake_up_state(task, TASK_INTERRUPTIBLE))
kick_process(t);
}
}

// called by exit_to_user_mode_loop() if ti_work & _TIF_NOTIFY_SIGNAL
void tracehook_notify_signal(regs)
{
clear_thread_flag(TIF_NOTIFY_SIGNAL);
smp_mb__after_atomic();
if (unlikely(current->task_works))
task_work_run();
}

This way task_work_run() doesn't need to clear TIF_NOTIFY_SIGNAL and it can
have more users.

What do you think?

Oleg.

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