Date: Thu, 11 Jun 2009 11:50:15 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, ego@in.ibm.com,
       rusty@rustcorp.com.au, mingo@elte.hu, linux-kernel@vger.kernel.org,
       peterz@infradead.org, oleg@redhat.com, dipankar@in.ibm.com
Subject: Re: [PATCH -mm resend] cpuhotplug: introduce try_get_online_cpus()
	take 3
Message-ID: <20090611185014.GJ6727@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <4A27708C.6030703@cn.fujitsu.com> <20090605153714.GB6778@linux.vnet.ibm.com> <20090608041934.GB17979@in.ibm.com> <20090608142520.GA6961@linux.vnet.ibm.com> <4A2E506D.9090107@cn.fujitsu.com> <20090609123438.b936137e.akpm@linux-foundation.org> <20090609234757.GH16117@linux.vnet.ibm.com> <4A2F08D6.6060309@cn.fujitsu.com> <20090609184238.06b38c3e.akpm@linux-foundation.org> <4A30C346.8070406@cn.fujitsu.com>
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On Thu, Jun 11, 2009 at 04:41:42PM +0800, Lai Jiangshan wrote:
> Andrew Morton wrote:
> > 
> > I still think we should really avoid having to do this.  trylocks are
> > nasty things.
> > 
> > Looking at the above, one would think that a correct fix would be to fix
> > the bug in "thread 2": take the locks in the correct order?  As
> > try_get_online_cpus() doesn't actually have any callers, it's hard to
> > take that thought any further.
> 
> Sometimes, we can not reorder the locks' order.
> try_get_online_cpus() is really needless when no one uses it.
> 
> Paul's expedited RCU V7 may need it:
> http://lkml.org/lkml/2009/5/22/332
> 
> So this patch can be omitted when Paul does not use it.
> It's totally OK for me.

Although my patch does not need it in and of itself, if someone were
to hold a kernel mutex across synchronize_sched_expedited(), and also
acquire that same kernel mutex in a hotplug notifier, the deadlock that
Lai calls out would occur.

Even if no one uses synchronize_sched_expedited() in this manner, I feel
that it is good to explore the possibility of dealing with it.  As
Andrew Morton pointed out, CPU-hotplug locking is touchy, so on-the-fly
fixes are to be avoided if possible.

							Thanx, Paul
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