Re: [PATCH v1] CPU hotplug: active_reader not woken up in some cases - deadlock

From: Paul E. McKenney
Date: Mon Dec 08 2014 - 18:57:17 EST

On Mon, Dec 08, 2014 at 08:30:18PM +0100, David Hildenbrand wrote:
> > > active_writer is cleared while holding cpuhp_lock, so this should be safe,
> > > right?
> >
> > You lost me on that one. Don't we get to that piece of code precisely
> > because we don't hold any of the CPU-hotplug locks? If so, the
> > writer might well hold all the locks it needs, and might well change
> > cpu_hotplug.active_writer out from under us.
> >
> > What am I missing here?
> >
> > Thanx, Paul
> I was missing that cpuhp_lock_* are simply lockdep anotations ... it's
> getting late :)
> So you're right, we need to verify that we don't get a 0 on the second access.

All you should need to do is to do something like this:

awp = ACCESS_ONCE(cpu_hotplug.active_writer);
if (awp)

That way you only have one access, and the check and wake_up_process()
are guaranteed to be consistent.

> Will send an updated version soon.

Sounds good!

Thanx, Paul

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