netdev - Re: [PATCH] drivers/net/b44: Change to non-atomic bit operations

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Message-ID: <20191225122424.5bc18036@hermes.lan>
Date:   Wed, 25 Dec 2019 12:24:24 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     David Miller <davem@...emloft.net>, michael.chan@...adcom.com,
        netdev@...r.kernel.org, tglx@...utronix.de, luto@...nel.org,
        peterz@...radead.org, tony.luck@...el.com, David.Laight@...LAB.COM,
        ravi.v.shankar@...el.com
Subject: Re: [PATCH] drivers/net/b44: Change to non-atomic bit operations

On Tue, 24 Dec 2019 17:10:20 -0800
Fenghua Yu <fenghua.yu@...el.com> wrote:

> On Tue, Dec 24, 2019 at 04:18:26PM -0800, David Miller wrote:
> > From: Fenghua Yu <fenghua.yu@...el.com>
> > Date: Fri, 20 Dec 2019 15:29:11 -0800
> >   
> > > On x86, accessing data across two cache lines in one atomic bit
> > > operation (aka split lock) can take over 1000 cycles.  
> > 
> > This happens during configuration of WOL, nobody cares that the atomic
> > operations done in this function take 1000 cycles each.
> > 
> > I'm not applying this patch.  It is gratuitous, and the commit message
> > talks about "performance" considuations (cycle counts) that completely
> > don't matter here.
> > 
> > If you are merely just arbitrarily trying to remove locked atomic
> > operations across the tree for it's own sake, then you should be
> > completely honest about that in your commit message.  
> 
> We are enabling split lock in the kernel (by default):
> https://lkml.org/lkml/2019/12/12/1129
> 
> After applying the split lock detection patch, the set_bit() in b44.c
> may cause split lock and kernel dies.
> 
> So should I change the commit message to add the above info?
> 
> Thanks.
> 
> -Fenghua

Why not just make pwol_pattern aligned and choose the right word to do
the operation on?