Re: [PATCH v4 2/6] perf: Enqueue SIGTRAP always via task_work.

[Frederic Weisbecker]

Le Thu, Dec 05, 2024 at 11:28:41AM +0100, Oleg Nesterov a écrit :
> On 12/05, Frederic Weisbecker wrote:
> >
> > Le Thu, Dec 05, 2024 at 10:20:16AM +0100, Oleg Nesterov a écrit :
> >
> > > > Looking at task_work, it seems that most enqueues happen to the current task.
> > > > AFAICT, only io_uring() does remote enqueue. Would it make sense to have a light
> > > > version of task_work that is only ever used by current? This would be a very
> > > > simple flavour with easy queue and cancellation without locking/atomics/RmW
> > > > operations.
> > >
> > > Perhaps, but we also need to avoid the races with task_work_cancel() from
> > > another task. I mean, if a task T does task_work_add_light(work), it can race
> > > with task_work_cancel(T, ...) which can change T->task_works on another CPU.
> >
> > I was thinking about two different lists.
> 
> OK... but this needs more thinking/discussion.

Sure.

> 
> > Another alternative is to maintain another head that points to the
> > head of the executing list. This way we can have task_work_cancel_current()
> > that completely cancels the work. That was my initial proposal here and it
> > avoids the lock/xchg for each work:
> >
> > https://lore.kernel.org/all/Zx-B0wK3xqRQsCOS@localhost.localdomain/
> 
> Thanks... Heh, I thought about something like this too ;) Although I thought
> that we need a bit more to implement task_work_cancel_sync(). But this is
> another story.

So which way do you prefer do solve the initial problem?

> 
> > > Hmm. I just noticed that task_work_run() needs a simple fix:
> > >
> > > 	--- x/kernel/task_work.c
> > > 	+++ x/kernel/task_work.c
> > > 	@@ -235,7 +235,7 @@
> > > 			raw_spin_unlock_irq(&task->pi_lock);
> > >
> > > 			do {
> > > 	-			next = work->next;
> > > 	+			next = READ_ONCE(work->next);
> > > 				work->func(work);
> > > 				work = next;
> > > 				cond_resched();
> > >
> > > Perhaps it makes sense before the patch from Sebastian even if that patch
> > > removes this do/while loop ?
> >
> > Hmm, can work->next be modified concurrently here?
> 
> work->func(work) can, say, do kfree(work) or do another task_work_add(X,
> work).

Right but then isn't it serialized program order, from the compiler point of view?

Thanks.

> 
> Oleg.
>