Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping duringidle

From: Jon Hunter
Date: Fri May 29 2009 - 21:01:23 EST

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.


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