Date: Wed, 13 Jan 2010 12:07:23 +0100
From: Heiko Carstens <heiko.carstens@de.ibm.com>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: linux-kernel@vger.kernel.org,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Steven Rostedt <rostedt@goodmis.org>, Oleg Nesterov <oleg@redhat.com>,
       Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
       akpm@linux-foundation.org, josh@joshtriplett.org, tglx@linutronix.de,
       Valdis.Kletnieks@vt.edu, dhowells@redhat.com, laijs@cn.fujitsu.com,
       dipankar@in.ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
	barrier (v5)
Message-ID: <20100113110723.GA4181@osiris.boeblingen.de.ibm.com>
References: <20100113013757.GA29314@Krystal>
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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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