jiffies wrap question...

jiffies wrap question...

[Jeff Garzik]

The following is a piece of code from the latter half of
schedule_timeout, in kernel/sched.c.  Is it possible that
schedule_timeout could return an incorrect value, if the jiffy value
wraps between the first and last lines shown below.

        expire = timeout + jiffies;

        init_timer(&timer);
        timer.expires = expire;
        timer.data = (unsigned long) current;
        timer.function = process_timeout;

        add_timer(&timer);
        schedule();
        del_timer_sync(&timer);

        timeout = expire - jiffies;


-- 
Jeff Garzik             |
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Re: jiffies wrap question...

[Philipp Rumpf]

On Fri, Nov 10, 2000 at 02:21:28AM -0500, Jeff Garzik wrote:
> The following is a piece of code from the latter half of
> schedule_timeout, in kernel/sched.c.  Is it possible that
> schedule_timeout could return an incorrect value, if the jiffy value
> wraps between the first and last lines shown below.

let's say the first line happens n ticks before the wrap and the second,
m ticks afterwards.

>         expire = timeout + jiffies;

expire = timeout + 2^32/2^64 - n = timeout - n

>         timeout = expire - jiffies;

timeout = expire - m = timeout - n - m = timeout - (n+m)

(n+m) is the time that actually passed while we were asleep, and timeout
has the correct value (timeout - ticks that happened)>
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