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

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Date:	Mon, 8 Dec 2014 20:30:18 +0100
From:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
To:	"Paul E. McKenney" <paulmck@...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

> > 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.

Will send an updated version soon.

Thanks!

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