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SubjectRe: workqueue deadlock

* Andrew Morton <akpm@osdl.org> wrote:

> This is actually not cpu-hotplug safe ;)
>
> > > > {
> > > > int cpu = raw_smp_processor_id();
> > > > /*
> > > > * Interrupts/softirqs are hotplug-safe:
> > > > */
> > > > if (in_interrupt())
> > > > return;
> > > > if (current->hotplug_depth++)
> > > > return;
>
> <preempt, cpu hot-unplug, resume on different CPU>
>
> > > > current->hotplug_lock = &per_cpu(hotplug_lock, cpu);
>
> <use-after-free>
>
> > > > mutex_lock(current->hotplug_lock);
>
> And it sleeps, so we can't use preempt_disable().

i explained it in the other mail - this is the 'read' side. The 'write'
side (code actually wanting to /do/ a CPU hotplug state transition) has
to take /all/ these locks before it can take a CPU down.

so this is still a global CPU hotplug lock, but made scalable.

Ingo
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