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

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Date:	Thu, 10 Oct 2013 14:43:27 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	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 Thu, 10 Oct 2013 11:10:35 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:
     .. now we can free all the percpu data and kill the CPU ..
> 
> without any locking anywhere - not stop-machine, not anything. If
> somebody is doing a "for_each_cpu()" (under just a regular
> rcu_read_lock()) and they see the bit set while it's going down, who
> cares? The CPU is still there, the data is accessible..

The problem is that rcu_read_lock() requires preemption disabled unless
you are using the preemptable rcu tree version. There's always
srcu_read_lock() but that's not so free. It's basically the same as
what Peter is doing.

There's places in the kernel that does for_each_cpu() that I'm sure you
don't want to disable preemption for. Especially when you start having
4096 CPU machines!

-- Steve
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