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

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Message-ID: <20131014092355.GM5790@linux.vnet.ibm.com>
Date:	Mon, 14 Oct 2013 02:23:55 -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 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...

> > 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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