From: Ingo Molnar <mingo@elte.hu>

It unifies the functionality of add_timer() and mod_timer(), and makes any
combination of the timer API calls completely SMP-safe.  del_timer() is still
not using the timer lock.

this patch fixes the only timer bug in 2.6 i'm aware of: the del_timer_sync()
+ add_timer() combination in kernel/itimer.c is buggy.  This was correct code
in 2.4, because there it was safe to do an add_timer() from the timer handler
itself, parallel to a del_timer_sync().

If we want to make this safe in 2.6 too (which i think we want to) then we
have to make add_timer() almost equivalent to mod_timer(), locking-wise.  And
once we are at this point i think it's much cleaner to actually make
add_timer() a variant of mod_timer().  (There's no locking cost for
add_timer(), only the cost of an extra branch.  And we've removed another
commonly used function from the icache.)