Jiffy based timers/timeouts can expire too soon.

[David Vrabel]

Hi,

Jiffy based timers and timeouts can expire too soon because the timer 
interrupt accounts for lost ticks and can increment jiffies by more than 1.

Consider the following:

    unsigned long timeout = jiffies + 1;

   <--- timer interrupt here:
        jiffies += 2 (i.e., catching up one missed interrupt)

   if (time_after(jiffies, timeout))
	/* but 1 tick worth of time hasn't (necessarily) elapsed */

This was originally observed on an ARM platform[1] but the i386 timer 
interrupt appears to behave in a similar way.

Is this solution here to:

1. Not use jiffies for timers/timeouts with only a few ticks?

or

2. Have two independant "jiffies": the existing one which is used for 
the wallclock only; and one which counts the number of timer interrupts 
and will guarantee that timers don't expire prematurely?

or

3. Something else?

David Vrabel

[1] 
http://lists.arm.linux.org.uk/pipermail/linux-arm-kernel/2004-December/025695.html
--
David Vrabel, Design Engineer

Arcom, Clifton Road           Tel: +44 (0)1223 411200 ext. 3233
Cambridge CB1 7EA, UK         Web: http://www.arcom.com/
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