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Date: Thu, 10 Oct 2013 14:43:27 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>, Oleg Nesterov <oleg@redhat.com>,
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        Johannes Weiner <hannes@cmpxchg.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        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 Thu, 10 Oct 2013 11:10:35 -0700
Linus Torvalds <torvalds@linux-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