Re: [PATCH v3 for-4.19 1/3] x86/irq: deal with old_cpu_mask for interrupts in movement in fixup_irqs()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jun 17, 2024 at 03:18:42PM +0200, Jan Beulich wrote:
> On 13.06.2024 18:56, Roger Pau Monne wrote:
> > Given the current logic it's possible for ->arch.old_cpu_mask to get out of
> > sync: if a CPU set in old_cpu_mask is offlined and then onlined
> > again without old_cpu_mask having been updated the data in the mask will no
> > longer be accurate, as when brought back online the CPU will no longer have
> > old_vector configured to handle the old interrupt source.
> > 
> > If there's an interrupt movement in progress, and the to be offlined CPU 
> > (which
> > is the call context) is in the old_cpu_mask clear it and update the mask, 
> > so it
> > doesn't contain stale data.
> 
> Perhaps a comma before "clear" might further help reading. Happy to
> add while committing.

Maybe, I'm trying to think of other ways to word the sentence in order
to make is simpler, but I'm out of ideas.

> > Note that when the system is going down fixup_irqs() will be called by
> > smp_send_stop() from CPU 0 with a mask with only CPU 0 on it, effectively
> > asking to move all interrupts to the current caller (CPU 0) which is the 
> > only
> > CPU to remain online.  In that case we don't care to migrate interrupts that
> > are in the process of being moved, as it's likely we won't be able to move 
> > all
> > interrupts to CPU 0 due to vector shortage anyway.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks, Roger.