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

From: Mathieu Desnoyers <[email-masked]>
Date: Thu, 5 Oct 2017 21:44:32 +0000 (UTC)

----- On Oct 5, 2017, at 12:21 PM, Peter Zijlstra [email-masked] wrote:

> 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 ?
> 
> If you do CLONE_VM without CLONE_THREAD you'll end up sharing the mm but
> you'll not be part of thread_head, so the for_each_thread() iteration
> will not find the task.

Excellent point. Please see the follow up RFC patch I posted taking care of
this matter.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com