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SubjectRe: [patch] jiffies wraparound [Re: 2.1.125 Show stopper list: Draft]

On Sun, 18 Oct 1998, Alan Cox wrote:

> > It's a beauty wart, it's been there forever, and we know that machines
> > actually tend to survive it fine. So far I have _one_ person who has
> 1.2 survives mostly, 2.0 survives mostly, 2.1.x doesnt survive

hm, i've just reviewed the timer code from this angle again, and there is
a suspicious piece of code:

} else if (expires < timer_jiffies) {
/* can happen if you add a timer with expires == jiffies,
* or you set a timer to go off in the past
insert_timer(timer, tv1.vec, tv1.index);
=====> } else if (idx < 0xffffffffUL) {
int i = (expires >> (TVR_BITS + 3 * TVN_BITS)) & TVN_MASK;
insert_timer(timer, tv5.vec, i);
} else {
/* Can only get here on architectures with 64-bit jiffies */
timer->next = timer->prev = timer;

shouldnt the (idx < 0xffffffffUL) condition be: (idx <= 0xffffffffUL)?

eg. if we have 'timer_jiffies == 0x0UL' (just wrapped around), and
'expires == 0xffffffffUL', then we have 'idx == 0xffffffffUL', but
(expires < timer_jiffies) is not true on a 32 bit box. Thus we get into
the last branch, which creates a timer with an infinit timeout ...

but this shouldnt happen too often i think when we just randomly wrap
jiffies (without heavy load that delays the processing of timer bhs), so i
guess there is something else going on.

-- mingo

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