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

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Date:   Tue, 24 Dec 2019 18:23:07 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     fenghua.yu@...el.com
Cc:     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

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

> 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?

You're asking me if you should make your commit message accurate?