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

[Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>]

----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx 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.

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

Thanks,

Mathieu

> 
> > +}

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