linux-kernel - Re: [PATCH for 4.16 04/10] membarrier: provide SHARED

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Date:   Tue, 16 Jan 2018 19:20:33 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrew Hunter <ahh@...gle.com>,
        Maged Michael <maged.michael@...il.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>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Greg Hackmann <ghackmann@...gle.com>,
        Will Deacon <will.deacon@....com>,
        David Sehr <sehr@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org
Subject: Re: [PATCH for 4.16 04/10] membarrier: provide SHARED_EXPEDITED
 command (v2)

On Mon, 15 Jan 2018, Mathieu Desnoyers wrote:
> +static int membarrier_shared_expedited(void)
> +{
> +	int cpu;
> +	bool fallback = false;
> +	cpumask_var_t tmpmask;
> +
> +	if (num_online_cpus() == 1)
> +		return 0;
> +
> +	/*
> +	 * Matches memory barriers around rq->curr modification in
> +	 * scheduler.
> +	 */
> +	smp_mb();	/* system call entry is not a mb. */
> +
> +	/*
> +	 * Expedited membarrier commands guarantee that they won't
> +	 * block, hence the GFP_NOWAIT allocation flag and fallback
> +	 * implementation.
> +	 */
> +	if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
> +		/* Fallback for OOM. */
> +		fallback = true;
> +	}
> +
> +	cpus_read_lock();
> +	for_each_online_cpu(cpu) {
> +		struct task_struct *p;
> +
> +		/*
> +		 * Skipping the current CPU is OK even through we can be
> +		 * migrated at any point. The current CPU, at the point
> +		 * where we read raw_smp_processor_id(), is ensured to
> +		 * be in program order with respect to the caller
> +		 * thread. Therefore, we can skip this CPU from the
> +		 * iteration.
> +		 */
> +		if (cpu == raw_smp_processor_id())
> +			continue;
> +		rcu_read_lock();
> +		p = task_rcu_dereference(&cpu_rq(cpu)->curr);
> +		if (p && p->mm && (atomic_read(&p->mm->membarrier_state) &
> +				   MEMBARRIER_STATE_SHARED_EXPEDITED)) {

This does not make sense vs. the documentation:

> + * @MEMBARRIER_CMD_SHARED_EXPEDITED:
> + *                          Execute a memory barrier on all running threads
> + *                          part of a process which previously registered
> + *                          with MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED.

This should say:

> + *                          Execute a memory barrier on all running threads
> + *                          of all processes which previously registered
> + *                          with MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED.

And I really have to ask whether this should be named _GLOBAL_ instead of
_SHARED_.

Hmm?

Thanks,

	tglx