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

[Andi Kleen]

On Sat, Jan 15, 2005 at 04:09:19PM +1100, Rusty Russell wrote:
> On Sat, 2005-01-15 at 05:09 +0100, Andi Kleen wrote:
> > Fix boot up SMP race in timer setup on i386/x86-64.
> > 
> > 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.
> > 
> > The race only happens when a CPU takes a long time to boot
> > (e.g. very slow console output with debugging enabled).
> > 
> > To fix I added a new cpu notifier notifier command CPU_UP_PREPARE_EARLY
> > that is called before the secondary CPU is started. timer.c
> > uses that now to initialize the per CPU timers early before
> > the other CPU runs any Linux code.
> 
> Andi, that's horrible.  I suspect you know it's horrible and were hoping
> someone would fix it properly.  The semantics of CPU_UP_PREPARE are
> supposed to do this already.

I shortly considered redoing the boot process, but then it looked 
too risky to me. 

e.g. I guess on x86-64 it wouldn't be that difficult, just a bit of work,
but on i386 with all the weird hardware it could be quite destabilizing.
But doing it on x86-64 only is not a good solution.


> 
> The cause of this bug is that (1) i386 and x86_64 actually bring the
> secondary CPUs up at boot before the core code officially brings them up
> using cpu_up(), after the appropriate callbacks, and (2) they call into
> core code tp process timer interrupts before they've been officially
> brought up.
> 
> The former is because I just added a shim rather than rewriting the x86
> boot process, because it would have broken too much.  The fix is do the
> boot process properly, or to suppress the call to do_timer before the
> CPU is actually "up".

I don't see a better short term solution.

If you had done it properly in 2.5 it would be working and tested
by now ;-) , but doing it in the middle of 2.6 would seem a bit misplaced
to me.

-Andi
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