linux-kernel - Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

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Message-ID: <20131015010121.GB13655@linux.vnet.ibm.com>
Date:	Mon, 14 Oct 2013 18:01:21 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

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

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