On Thu, Oct 05, 2017 at 02:12:50PM +0200, Peter Zijlstra 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.
Also, for easier reading I would suggest putting { } around the block.
> > + 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?
>
> > +}
