linux-kernel - Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

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Message-ID: <20170814184711.GL6524@worktop.programming.kicks-ass.net>
Date:   Mon, 14 Aug 2017 20:47:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org,
        Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to
 improve performance on some archs

On Mon, Aug 14, 2017 at 01:01:22PM +0100, Will Deacon wrote:
> Yeah, that's right, you can't use the STXR status flag to create control
> dependencies.

Just for my elucidation; you can't use it to create a control dependency
on the store, but you can use it to create a control dependency on the
corresponding load, right?

Now, IIRC, we've defined control dependencies as being LOAD->STORE
ordering, so in that respect nothing is lost. But maybe we should
explicitly mention that if the LOAD is part of an (otherwise) atomic RmW
the STORE is not constrained.