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Date: Mon, 14 Oct 2013 18:01:21 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2
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On Mon, Oct 14, 2013 at 02:23:55AM -0700, Paul E. McKenney wrote:
> On Mon, Oct 14, 2013 at 11:05:08AM +0200, Peter Zijlstra wrote:
> > On Sat, Oct 12, 2013 at 07:06:56PM +0200, Oleg Nesterov wrote:
> > > it even disables irqs, so this should always imply rcu_read_lock() with
> > > any implementation, 
> > 
> > Not so; I could make an RCU implementation that drives the state machine
> > from rcu_read_unlock(). Such an implementation doesn't need the
> > interrupt driven poll-state driver we currently have and could thus
> > subvert that assumption :-)
> > 
> > Then again, there's a good reason PaulMck didn't pick this
> > implementation.
> 
> True enough, but there really are some out-of-tree RCU implementations
> that do take this approach and where disabling interrupts would not
> block preemptible RCU.  So please do not rely on this implementation
> detail.  You never know...

Actually, the current implementation of SRCU is not blocked by CPUs
disabling interrupts!

							Thanx, Paul

> > > In fact I do not even understand why getaffinity() doesn't simply
> > > return ->cpus_allowed, but this is off-topic.
> > 
> > Yeah, me neither :-(, it always surprises me. But changing it is likely
> > to break stuff so there we are.
> 
> I know that feeling...
> 
> 							Thanx, Paul