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

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Date:   Thu, 5 Oct 2017 14:24:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, Boqun Feng <boqun.feng@...il.com>,
        Andrew Hunter <ahh@...gle.com>,
        Maged Michael <maged.michael@...il.com>, gromer@...gle.com,
        Avi Kivity <avi@...lladb.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Watson <davejwatson@...com>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Nicholas Piggin <npiggin@...il.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited
 private command

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?
> 
> > +}