Subject: Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

From: Andrea Parri <[email-masked]>
Date: Fri, 6 Oct 2017 00:02:14 +0200

On Thu, Oct 05, 2017 at 04:02:06PM +0000, Mathieu Desnoyers wrote:
> ----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra [email-masked] wrote:
> 
> > On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:
> >> diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
> >> new file mode 100644
> >> index 000000000000..b0d79a5f5981
> >> --- /dev/null
> >> +++ b/arch/powerpc/kernel/membarrier.c
> >> @@ -0,0 +1,45 @@
> > 
> >> +void membarrier_arch_register_private_expedited(struct task_struct *p)
> >> +{
> >> +	struct task_struct *t;
> >> +
> >> +	if (get_nr_threads(p) == 1) {
> >> +		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> >> +		return;
> >> +	}
> >> +	/*
> >> +	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
> >> +	 * fork is protected by siglock.
> >> +	 */
> >> +	spin_lock(&p->sighand->siglock);
> >> +	for_each_thread(p, t)
> >> +		set_ti_thread_flag(task_thread_info(t),
> >> +				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> > 
> > I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.
> 
> The intent here is to hold the sighand siglock to provide mutual
> exclusion against invocation of membarrier_fork(p, clone_flags)
> by copy_process().
> 
> copy_process() grabs spin_lock(&current->sighand->siglock) for both
> CLONE_THREAD and not CLONE_THREAD flags.
> 
> What am I missing here ?
> 
> > 
> >> +	spin_unlock(&p->sighand->siglock);
> >> +	/*
> >> +	 * Ensure all future scheduler executions will observe the new
> >> +	 * thread flag state for this process.
> >> +	 */
> >> +	synchronize_sched();
> > 
> > This relies on the flag being read inside rq->lock, right?
> 
> Yes. The flag is read by membarrier_arch_switch_mm(), invoked
> within switch_mm_irqs_off(), called by context_switch() before
> finish_task_switch() releases the rq lock.

I fail to graps the relation between this synchronize_sched() and rq->lock.

(Besides, we made no reference to rq->lock while discussing:

  https://github.com/paulmckrcu/litmus/commit/47039df324b60ace0cf7b2403299580be730119b
  replace membarrier_arch_sched_in with switch_mm_irqs_off )

Could you elaborate?

  Andrea

> 
> Is the comment clear enough, or do you have suggestions for
> improvements ?

> Thanks,
> Mathieu
> > > +}
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com