Hi Patrick,

I think that I found something interesting reading kernel/timer.c some 
weeks ago, well I would like to know about this idea. The comment above 
mod_timer speaks by itself...

 * mod_timer(timer, expires) is equivalent to:
 *
 *     del_timer(timer); timer->expires = expires; add_timer(timer);
 *
 * Note that if there are multiple unserialized concurrent users of the
 * same timer, then mod_timer() is the only safe way to modify the timeout,
 * since add_timer() cannot modify an already running timer.
 *
 * The function returns whether it has modified a pending timer or not.
 * (ie. mod_timer() of an inactive timer returns 0, mod_timer() of an
 * active timer returns 1.)

so I think that you could use mod_timer in 2.6 and remove that 
WRITE_LOCK when updating the timer, because it's there to serialize 
timer updates and now mod_timer controls this possible situation. I 
don't have a smp machine here but this week I'll look for a platform to 
test it.

If I'm missing something, please let me know.

regards,
Pablo