linux-kernel - Re: [PATCH v3] CPU hotplug: active_writer not woken up in some cases

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Message-ID: <20141210031809.GO25340@linux.vnet.ibm.com>
Date:	Tue, 9 Dec 2014 19:18:09 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, heiko.carstens@...ibm.com,
	borntraeger@...ibm.com, rafael.j.wysocki@...el.com,
	peterz@...radead.org, bp@...e.de, jkosina@...e.cz
Subject: Re: [PATCH v3] CPU hotplug: active_writer not woken up in some	cases
 - deadlock

On Wed, Dec 10, 2014 at 01:26:19AM +0100, Oleg Nesterov wrote:
> On 12/09, Paul E. McKenney wrote:
> >
> > Would wait_event()/wake_up() work for the wakeup-writer case?
> 
> Yes, and in this case we could probably kill this puts_pending logic
> and avoid cpu_hotplug.lock in put_online_cpus() altogether? Can't we
> just make cpu_hotplug.refcount atomic_t?

Seems like that should be possible.  That would certainly simplify the
wakeup logic from put_online_cpus().  It might even be possible to
avoid acquiring cpu_hotplug.lock in put_online_cpus(), though that
would of course require more luck than anyone deserves.

> Anyway, this makes me think again that this code should use percpu_rwsem.
> Perhaps I'll try to make a patch next week...
> 
> (we need down_write_recursive_readers(), and probably rcusync patches).

Careful!  You might end up re-introducing the deadlock that I used
puts_pending to get rid of.  ;-)

							Thanx, Paul

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