Re: [PATCH]Timer list init is done AFTER use

[george anzinger]

Andrew Morton wrote:
> 
> george anzinger wrote:
> >
> > Andrew Morton wrote:
> > >
> > > george anzinger wrote:
> > > >
> > > > On SMP systems the timer list init is done by way of a
> > > > cpu_notifier call.  This has two problems:
> > > >
> > > > 1.) Timers are started WAY before the cpu_notifier call
> > > > chain is executed.  In particular the console blanking timer
> > > > is deleted and inserted every time printk() is called.  That
> > > > this does not fail is only because the kernel has yet to
> > > > protect location zero.
> > >
> > > But init_timers() directly calls timer_cpu_notify(), which directly
> > > calls init_timers_cpu().
> > >
> > > So your patch appears to be a no-op for the boot CPU.
> >
> > That is correct.  The problem is when cpu_init is called for
> > the secondary cpus.  It almost immediately calls printk.
> 
> OK.  So until that CPU sees it bit come on in smp_commenced_mask()
> it's not allowed to assume that it is running yet.
> 
> > ...
> > My comments here are a wonderment if
> > this is the right thing to do when doing a hot swap of the
> > cpu.  I sort of doubt that this is correct.
> 
> I agree.  And from a quick read it does seem that ia32 is
> doing the right thing apart from calling printk.
> 
> I don't think we should make changes to the timer code because
> who knows what assumptions other console drivers could be making?
> 
> I don't think we should carefully remove all printk() calls because
> printk() is supposed to be robust, and always callable.
> 
> The logical thing is to implement arch_consoles_callable().  Does
> this look workable?

I am not sure.  The first question is when does the online
bit get set for cpu 0.  The next is that it does inhibit a
rather large block of printks.  Is this ok?

Mind you, I have not tried it yet...

-g
> 
> --- 25/kernel/printk.c~ga       Fri Dec 20 11:32:05 2002
> +++ 25-akpm/kernel/printk.c     Fri Dec 20 11:33:14 2002
> @@ -43,7 +43,11 @@
>  #define LOG_BUF_MASK   (LOG_BUF_LEN-1)
> 
>  #ifndef arch_consoles_callable
> -#define arch_consoles_callable() (1)
> +/*
> + * Some console drivers may assume that per-cpu resources have been allocated.
> + * So don't allow them to be called by this CPU until it is officially up.
> + */
> +#define arch_consoles_callable() cpu_online(smp_processor_id())
>  #endif
> 
>  /* printk's without a loglevel use this.. */
> 

-- 
George Anzinger   [e-mail]
High-res-timers: 
http://sourceforge.net/projects/high-res-timers/
Preemption patch:
http://www.kernel.org/pub/linux/kernel/people/rml