linux-kernel - Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v5)

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Date:	Wed, 13 Jan 2010 12:07:23 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	josh@...htriplett.org, tglx@...utronix.de, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, laijs@...fujitsu.com, dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
	barrier (v5)

On Tue, Jan 12, 2010 at 08:37:57PM -0500, Mathieu Desnoyers wrote:
> +static void membarrier_retry(void)
> +{
> +	struct mm_struct *mm;
> +	int cpu;
> +
> +	for_each_cpu(cpu, mm_cpumask(current->mm)) {
> +		spin_lock_irq(&cpu_rq(cpu)->lock);
> +		mm = cpu_curr(cpu)->mm;
> +		spin_unlock_irq(&cpu_rq(cpu)->lock);
> +		if (current->mm == mm)
> +			smp_call_function_single(cpu, membarrier_ipi, NULL, 1);
> +	}

You would need to disable cpu unplug operations while doing this
or you might end up sending IPIs to offline cpus.

> +	cpumask_copy(tmpmask, mm_cpumask(current->mm));
> +	preempt_disable();
> +	cpumask_clear_cpu(smp_processor_id(), tmpmask);
> +	for_each_cpu(cpu, tmpmask) {
> +		spin_lock_irq(&cpu_rq(cpu)->lock);
> +		mm = cpu_curr(cpu)->mm;

This might access the rq of an offline cpu.
But maybe it's intended since offline cpus "run" idle?
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