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Date:	Mon, 07 Feb 2011 09:12:54 +0100
From:	Mike Galbraith <efault@....de>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Milton Miller <miltonm@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	akpm@...ux-foundation.org, Anton Blanchard <anton@...ba.org>,
	xiaoguangrong@...fujitsu.com, mingo@...e.hu, jaxboe@...ionio.com,
	npiggin@...il.com, JBeulich@...ell.com, rusty@...tcorp.com.au,
	torvalds@...ux-foundation.org, benh@...nel.crashing.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3 v2] call_function_many: fix list delete vs add race

On Tue, 2011-02-01 at 20:17 -0800, Paul E. McKenney wrote:

FWIW, my red headed stepchild (.32 xen cluster beast) with..
   smp-smp_call_function_many-fix-SMP-race (6dc1989)
   smp-consolidate-writes-in-smp_call_function_interrupt (225c8e0)
   smp-smp_call_function_many-fix-list-delete-vs-add-race (V2)
   smp-smp_call_function_many-handle-concurrent-clearing-of-mask (V2)
   smp-generic_smp_call_function_interrupt-additional-memory-order-tightening (below)
..has not experienced any IPI problems lately, nor have I been able to
trigger anything beating up my box running normal x64_64 kernels.

That's not saying much since my IPI woes were only the concurrent
clearing variety, just letting you know that these patches have received
(and are receiving) hefty thumpage.

> ------------------------------------------------------------------------
> 
> smp_call_function: additional memory-order tightening.
> 
> The csd_lock() and csd_unlock() interaction guarantees that the
> smp_call_function_many() function sees the results of interactions
> with prior incarnations of the callback, so the check is not needed.
> Instead, tighter memory ordering is required in the companion
> generic_smp_call_function_interrupt() function to ensure proper
> interaction with partially initialized callbacks.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 064bb6e..e091905 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -182,7 +182,7 @@ void generic_smp_call_function_interrupt(void)
>  
>  	/*
>  	 * Ensure entry is visible on call_function_queue after we have
> -	 * entered the IPI. See comment in smp_call_function_many.
> +	 * entered the IPI. See comment in smp_call_function_many
>  	 * If we don't have this, then we may miss an entry on the list
>  	 * and never get another IPI to process it.
>  	 */
> @@ -209,13 +209,18 @@ void generic_smp_call_function_interrupt(void)
>  		if (!cpumask_test_cpu(cpu, data->cpumask))
>  			continue;
>  
> -		smp_rmb();
> +		smp_mb(); /* If we see our bit set above, we need to see */
> +			  /* all the processing associated with the prior */
> +			  /* incarnation of this callback. */
>  
>  		if (atomic_read(&data->refs) == 0)
>  			continue;
>  
> +		smp_rmb(); /* We need to read ->refs before we read either */
> +			   /* ->csd.func and ->csd.info. */
> +
>  		func = data->csd.func;			/* for later warn */
> -		data->csd.func(data->csd.info);
> +		func(data->csd.info);
>  
>  		/*
>  		 * If the cpu mask is not still set then func enabled


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