[Q] jiffies overflow & timers.

From: Ihar 'Philips' Filipau
Date: Mon Nov 17 2003 - 13:37:07 EST


Hello!

[ ver 2.4.18/22 ]

[ I feel strongly that is FAQ - so any ptr(RTFM)!=0 apreciated. ]

[ ldd2 covers this very sparsely - overflow case is not covered at all - just mentioned. Google - looks like I cannot find right keywords for this ... ]

I'm trying to find correct solution for case of jiffies overflow and standard kernel timers (./kernel/timer.c).

My module has to maintain list of timers. I cannot reuse directly struct timer_list - since it uses jiffies and jiffies do wrap on overflow.

I decided to use struct timeval & do_gettimeofday(). But still I have to handle case when next timer to expire will happend after jiffies will overflow.

So my question - how to detect that jiffies had overflown?

Is the following code is sufficient?
(Assuming that I will not try to set timer longer than (~0UL/(HZ)) seconds)

unsigned long
tv_get_next_expiring_jiffies( struct timeval *target_tv )
{
struct timeval curr_tv, timeout;
ulong dif_jif;

do_gettimeofday( &curr_tv );

/* timeout = curr_tv - target_tv */
tv_sub( &timeout, &curr_tv, target_tv );

dif_jif = tv_to_jiffies( &timeout ); /* assumption above. */

if (jiffies > ~0UL - dif_jif) {
/* will overflow
* so wait for overflow, then just reschedule
*/
return ~0UL;
} else {
/* will not */
return jiffies + dif_jif;
}
}

Is (~0UL) "safe harbour"? in other words - will this value reached? /me cannot find where jiffies is incremented... Dumd grep -r jiffies gives no results (no assignment, no taking of pointer, literally no matches in .S files - what I'm missing?) Because if this value will be reached and I will detect that jiffies == ~0UL I will have to "while(jiffies == ~0UL);" wait for overflow.

--
Ihar 'Philips' Filipau / with best regards from Saarbruecken.
-- _ _ _
"... and for $64000 question, could you get yourself |_|*|_|
vaguely familiar with the notion of on-topic posting?" |_|_|*|
-- Al Viro @ LKML |*|*|*|

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