linux-kernel - Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping during idle

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Date:	Fri, 29 May 2009 20:00:40 -0500
From:	Jon Hunter <jon-hunter@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	john stultz <johnstul@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping during
 idle

Jon Hunter wrote:
> +		 * Calculate the time delta for the next timer event.
> +		 * If the time delta exceeds the maximum time delta
> +		 * permitted by the current clocksource then adjust
> +		 * the time delta accordingly to ensure the
> +		 * clocksource does not wrap.
> +		 */
> +		time_delta = tick_period.tv64 * delta_jiffies;

Thinking about this more, although it is very unlikely, for 64-bit 
machines there is a chance that the above multiply could overflow if 
delta_jiffies is very large.

tick_period.tv64 should always be less than NSEC_PER_SEC and so you 
would need delta_jiffies to be greater than 2^32 to cause overflow. On a 
32-bit machine an unsigned long will not be greater than 2^32 as it is 
only 32-bits but this would be possible on a 64-bit machines.

So to be safe we should make sure that delta_jiffies is not greater than 
  NEXT_TIMER_MAX_DELTA (2^30 - 1) before doing the multiply. If you 
think that this is a valid concern, then I can re-work and re-post. 
Sorry for not catching this before.

Jon

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