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

Andrew Morton (akpm@digeo.com)
Fri, 20 Dec 2002 11:34:25 -0800


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?

--- 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.. */

_
-
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