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

[Oleg Nesterov <oleg@redhat.com>]

Sorry, currently I don't have time to even read the emails from lkml.

Plus I wasn't cc'ed so I don't understand the intent at all, but ...

On 11/08, Frederic Weisbecker wrote:
>
> > @@ -232,13 +222,24 @@ void task_work_run(void)
> >  		 * But it can remove another entry from the ->next list.
> >  		 */
> >  		raw_spin_lock_irq(&task->pi_lock);
> > +		do {
> > +			head = NULL;
> > +			if (work) {
> > +				head = READ_ONCE(work->next);
> > +			} else {
> > +				if (task->flags & PF_EXITING)
> > +					head = &work_exited;
> > +				else
> > +					break;
> > +			}
> > +		} while (!try_cmpxchg(&task->task_works, &work, head));
> >  		raw_spin_unlock_irq(&task->pi_lock);
>
> And having more than one task work should be sufficiently rare
> that we don't care about doing the locking + cmpxchg() for each
> of them pending.

Please see
https://lore.kernel.org/all/1440816150.8932.123.camel@edumazet-glaptop2.roam.corp.google.com/
and the whole thread.

I don't think raw_spin_lock_irq + cmpxchg for each work is a good
idea, but quite possibly I misunderstood this change.

Oleg.