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

[Andrew Morton <akpm@digeo.com>]

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.. */

_