linux-kernel - Re: [PATCH] tick: add check for the existence of broadcast clock event device

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Date:	Sat, 6 Jun 2009 11:24:50 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Feng Tang <feng.tang@...el.com>
cc:	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tick: add check for the existence of broadcast clock
 event device

On Fri, 5 Jun 2009, Feng Tang wrote:

> >From 2f076e1867c8bbb145b74d289358174644d9fed8 Mon Sep 17 00:00:00 2001
> From: Feng Tang <feng.tang@...el.com>
> Date: Fri, 5 Jun 2009 10:36:15 +0800
> Subject: [PATCH] tick: add check for the existence of broadcast clock event device
> 
> Some platform may have no broadcast clock event device, as it use always-on
> external timers for per-cpu timer and has no extra one for broadcast device.
> This check will secure the access to bc device when system get some boradcast
> on/off and enter/exit message
>
> Signed-off-by: Feng Tang <feng.tang@...el.com>
> ---
>  kernel/time/tick-broadcast.c |    8 +++++++-
>  1 files changed, 7 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
> index 118a3b3..110e0bc 100644
> --- a/kernel/time/tick-broadcast.c
> +++ b/kernel/time/tick-broadcast.c
> @@ -214,10 +214,13 @@ static void tick_do_broadcast_on_off(void *why)
>  
>  	spin_lock_irqsave(&tick_broadcast_lock, flags);
>  
> +	bc = tick_broadcast_device.evtdev;
> +	if (!bc)
> +		goto out;
> +

This check is not necessary because we check whether the percpu device
is affected by the stops in C3 madness _before_ we touch the broadcast
device.

	 if (!dev || !(dev->features & CLOCK_EVT_FEAT_C3STOP))
	    	  got out;

If your percpu devices are always on (not affected by C3 stop) then
you never dereference bc. So why do we need an extra check for !bc ?

Thanks,

	tglx
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