linux-kernel - Re: [PATCH v2] bit_spinlock: introduce smp_cond_load

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Date:   Tue, 6 Nov 2018 13:43:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Gao Xiang <gaoxiang25@...wei.com>
Cc:     Will Deacon <will.deacon@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Miao Xie <miaoxie@...wei.com>,
        Chao Yu <chao@...nel.org>
Subject: Re: [PATCH v2] bit_spinlock: introduce smp_cond_load_relaxed

On Tue, Nov 06, 2018 at 08:33:56PM +0800, Gao Xiang wrote:
> Hi Peter,
> 
> On 2018/11/6 20:27, Peter Zijlstra wrote:
> > On Tue, Nov 06, 2018 at 07:36:41PM +0800, Gao Xiang wrote:
> >> IMO, to use wrapped up function for the detailed scenario could be better than
> >> open-coded all the time (eg. do cpu_relax(); while(...)) since it could be
> >> optimizated even more for the specific architecture...
> > That's the whole point though; if this actually matters, you're doing it
> > wrong.
> 
> I cannot fully understand your point...Sorry about my English...
> 
> To the point, you mean it is much better to fix it as Will suggested before or
> leave the matter as it is since the performance of bit_spinlock itself doesn't matter?

Right, bit-spinlocks are terrible when contended. If the contended
behaviour of bit-spinlocks start to matter, you've lost already.