Date: Mon, 8 Dec 2014 20:30:18 +0100
From: David Hildenbrand <dahi@linux.vnet.ibm.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, heiko.carstens@de.ibm.com,
        borntraeger@de.ibm.com, rafael.j.wysocki@intel.com,
        peterz@infradead.org, srivatsa.bhat@linux.vnet.ibm.com,
        oleg@redhat.com, bp@suse.de, jkosina@suse.cz
Subject: Re: [PATCH v1] CPU hotplug: active_reader not woken up in some
 cases - deadlock
Message-ID: <20141208203018.24f23bdb@thinkpad-w530>
In-Reply-To: <20141208190803.GR25340@linux.vnet.ibm.com>
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	<20141208183134.GQ25340@linux.vnet.ibm.com>
	<20141208195814.663007dd@thinkpad-w530>
	<20141208190803.GR25340@linux.vnet.ibm.com>
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> > 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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