Re: [PATCH] i386/x86-64: Fix timer SMP bootup race

[Andi Kleen <ak@suse.de>]

On Fri, Jan 14, 2005 at 10:28:41PM -0800, Andrew Morton wrote:
> Andi Kleen <ak@suse.de> wrote:
> >
> > This fixes a long standing race in 2.6 i386/x86-64 SMP boot.
> >  The per CPU timers would only get initialized after an secondary
> >  CPU was running. But during initialization the secondary CPU would
> >  already enable interrupts to compute the jiffies. When a per 
> >  CPU timer fired in this window it would run into a BUG in timer.c
> >  because the timer heap for that CPU wasn't fully initialized.
> 
> Why don't we just not call calibrate_delay() on the secondaries?  It
> doesn't seem to do anything.  That way we can leave local interrupts
> disabled.

It's used for the "accumulative bogomips". To quote Alan: 

    /*
     * Allow the user to impress friends.
     */

But taking it away doesn't help because the timer startup on the BP
and the secondaries going into the idle loop isn't synchronized.
You could add a synchronization step, but it would be far more
complicated than fixing the ordering like I did.

-Andi