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

[Rusty Russell]

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.

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".

Sorry,
Rusty.
-- 
A bad analogy is like a leaky screwdriver -- Richard Braakman

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