Re: [RFC] mod_timer() helper functions?

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 17 May 2009 00:50:55 -0700 Chris Peterson <cpeterso@cpeterso.com> wrote:

> >> Reviewing the kernel's nearly one-thousand calls to mod_timer(), there
> >> are three basic patterns:
> >>
> >> __* multi-second timeouts
> >> __* millisecond timeouts
> >> __* +1 jiffie ASAP events
> >>
> 
> Is there a functional difference between the following "expire this
> timer ASAP" statements?
> 
> mod_timer(timer, jiffies + 1); /* 48 uses */
> mod_timer(timer, jiffies); /* 44 uses */
> mod_timer(timer, jiffies - 1); /* 6 uses */

That's something which has always worried me.  Lots of code does:

	mod_timer(timer, jiffies + 1);

(for varying values of "1").  What happens if this thread of control
then gets stalled for a couple of jiffies.  Say it gets preempted or
there's a long interrupt or whatever.  So there's a several-jiffy
interval between the caller evaluating jiffies+1 and the entry to
mod_timer().


>From my reading, we'll hit

		int i;
		/* If the timeout is larger than 0xffffffff on 64-bit
		 * architectures then we use the maximum timeout:
		 */
		if (idx > 0xffffffffUL) {
			idx = 0xffffffffUL;
			expires = idx + base->timer_jiffies;
		}
		i = (expires >> (TVR_BITS + 3 * TVN_BITS)) & TVN_MASK;
		vec = base->tv5.vec + i;

and the timer gets scheduled at some time in the far-distant future!

But this is such a glaring and huge problem that surely it cannot
exist.  But I don't know why not.

If the bug _does_ exist then mod_timer(timer, jiffies - 1) will set the
timer to go off in the far future.  mod_timer(timer, jiffies) will
usually make it go off real soon now, but it's scary.  mod_timer(timer,
jiffies + 1) is safer, but still vulnerable.