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

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Date:	Mon, 8 Dec 2014 12:47:17 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, heiko.carstens@...ibm.com,
	borntraeger@...ibm.com, rafael.j.wysocki@...el.com,
	peterz@...radead.org, srivatsa.bhat@...ux.vnet.ibm.com,
	oleg@...hat.com, bp@...e.de, jkosina@...e.cz
Subject: Re: [PATCH v1] CPU hotplug: active_reader not woken up in some cases
 - deadlock

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)
		wake_up_process(cpu_hotplug.active_writer);

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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