Date: Fri, 28 Jul 2017 13:57:02 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, Boqun Feng <boqun.feng@gmail.com>,
        Andrew Hunter <ahh@google.com>,
        Maged Michael <maged.michael@gmail.com>, gromer@google.com,
        Avi Kivity <avi@scylladb.com>, Michael Ellerman <mpe@ellerman.id.au>,
        Nicholas Piggin <npiggin@gmail.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Palmer Dabbelt <palmer@dabbelt.com>
Subject: Re: [RFC PATCH v2] membarrier: expedited private command
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On Fri, Jul 28, 2017 at 10:55:32AM +0200, Peter Zijlstra wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e9785f7aed75..33f34a201255 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2641,8 +2641,18 @@ static struct rq *finish_task_switch(struct task_struct *prev)
>  	finish_arch_post_lock_switch();
>  
>  	fire_sched_in_preempt_notifiers(current);
> +
> +	/*
> +	 * For CONFIG_MEMBARRIER we need a full memory barrier after the
> +	 * rq->curr assignment. Not all architectures have one in either
> +	 * switch_to() or switch_mm() so we use (and complement) the one
> +	 * implied by mmdrop()'s atomic_dec_and_test().
> +	 */
>  	if (mm)
>  		mmdrop(mm);
> +	else if (IS_ENABLED(CONFIG_MEMBARRIER))
> +		smp_mb();
> +
>  	if (unlikely(prev_state == TASK_DEAD)) {
>  		if (prev->sched_class->task_dead)
>  			prev->sched_class->task_dead(prev);
> 
> 

> a whole bunch of architectures don't in fact need this extra barrier at all.

In fact, I'm fairly sure its only PPC.

Because only ARM64 and PPC actually implement ACQUIRE/RELEASE with
anything other than smp_mb() (for now, Risc-V is in this same boat and
MIPS could be if they ever sort out their fancy barriers).

TSO archs use a regular STORE for RELEASE, but all their atomics imply a
smp_mb() and there are enough around to make one happen (typically
mm_cpumask updates).

Everybody else, aside from ARM64 and PPC must use smp_mb() for
ACQUIRE/RELEASE.

ARM64 has a super duper barrier in switch_to().

Which only leaves PPC stranded.. but the 'good' news is that mpe says
they'll probably need a barrier in switch_mm() in any case.