linux-kernel - Re: [RFC][PATCH 2/5] mutex: Use p->oncpu for the adaptive spin

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Date:	Fri, 17 Dec 2010 20:17:13 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Frank Rowand <frank.rowand@...sony.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 2/5] mutex: Use p->oncpu for the adaptive spin

On 12/16, Peter Zijlstra wrote:
>
> So yea, I think we can simplify all this even further, but we do need a
> smp_rmb() in between those two checks, hmm, and RCU.

I am not sure we need rmb() or even read_barrier_depends(), I think
the simple barrier() should be enough. Although rcu_dereference()
may be more clean.

We don't really care about the "correct" result of *owner. All we need,
we should ensure it is safe to dereference this address. If CPU itself
does something like "readahead" before we verified "lock->owner == owner"
under RCU lock, this shouldn't lead to the fault.

Oleg.

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