Re: [patch V2 3/5] posix-cpu-timers: Provide mechanisms to defer timer handling to task_work

Frederic Weisbecker
2020-07-23T01:03:20Z

On Fri, Jul 17, 2020 at 12:50:34AM +0200, Peter Zijlstra wrote:
> On Thu, Jul 16, 2020 at 10:19:26PM +0200, Thomas Gleixner wrote:
> > +static void __run_posix_cpu_timers(struct task_struct *tsk)
> > +{
> > + struct posix_cputimers *pct = &tsk->posix_cputimers;
> > +
> > + if (!test_and_set_bit(CPUTIMERS_WORK_SCHEDULED, &pct->flags))
> > + task_work_add(tsk, &pct->task_work, true);
> > +}
> > +
> > +static inline void posix_cpu_timers_enable_work(struct task_struct *tsk)
> > +{
> > + clear_bit(CPUTIMERS_WORK_SCHEDULED, &tsk->posix_cputimers.flags);
> /*
> * Ensure we observe everything before a failing test_and_set()
> * in __run_posix_cpu_timers().
> */
> smp_mb__after_atomic();
> > +}
>
> Such that when another timer interrupt happens while we run this, we're
> guaranteed to either see it, or get re-queued and thus re-run the
> function.

But each thread in the process enqueues its own task work and flips its
own flags. So if task A runs the task work and task B runs __run_posix_cpu_timers(),
they wouldn't be ordering against the same flags.