Re: workqueue deadlock

[Ingo Molnar <mingo@elte.hu>]

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

> > > > > > 	{
> > > > > > 		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.
> 
> Doesn't matter - the race is still there.
> 
> Well, not really, because we don't free the percpu data of offlined 
> CPUs, but we'd like to.
>
> And it's easily fixable by using a statically-allocated array.  That 
> would make life easier for code which wants to take this lock early in 
> boot too.

yeah.

but i think your freeze_processes() idea is even better - it would unify 
the suspend and the CPU-hotplug infrastructure even more. (and kprobes 
wants to use freeze_processes() too)

	Ingo