Re: [RFC][PATCH] timers fixes/improvements

[Oleg Nesterov]

Andrew Morton wrote:
> 
> Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> >
> > +void fastcall init_timer(struct timer_list *timer)
> >  +{
> >  +    timer->entry.next = NULL;
> >  +    timer->_base = &per_cpu(tvec_bases,
> >  +                    __smp_processor_id()).t_base;
> >  +    timer->magic = TIMER_MAGIC;
> >  +}
> 
> __smp_processor_id() is not implemented on all architectures.  I'll switch
> this to _smp_processor_id().

Wow, I did not know.

> It's a rather odd thing which you're doing there.  Why does a
> not-yet-scheduled timer need a ->_base?

Because all locking goes through timer_list->base->lock now.
That is why timer_list->lock can be deleted. The timer is
always locked via loc_timer_base().

timer->base == NULL only temporally when __mod_timer() does
while switching timer's base:
	base = lock_timer_base(timer);
	timer->base = NULL;
	unlock(base->lock);
		// Nobody can use this timer, lock_timer_base()
		// will spin waiting for ->base != 0
	lock(new_base->lock);
	timer->base = new_base;
	unlock(new_base);

So ->base == NULL means that timer itself is locked, not it's
base. That is why __mod_timer() do not need to hold 2 spinlocks
at once.

Oleg.
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