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

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Message-ID: <20131014090508.GW3081@twins.programming.kicks-ass.net>
Date:	Mon, 14 Oct 2013 11:05:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	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>,
	Paul McKenney <paulmck@...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 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.

> 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.
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