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Date:   Fri, 22 Nov 2019 10:36:32 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Andy Lutomirski <luto@kernel.org>,
        David Laight <David.Laight@aculab.com>,
        Ingo Molnar <mingo@kernel.org>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Tony Luck <tony.luck@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by
 kernel parameter
Message-ID: <20191122093632.GB4097@hirez.programming.kicks-ass.net>
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 <066A48B7-296F-4953-89A6-588509FC0303@amacapital.net>
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On Thu, Nov 21, 2019 at 01:01:08PM -0800, Andy Lutomirski wrote:
> 
> > On Nov 21, 2019, at 11:56 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > ﻿On Thu, Nov 21, 2019 at 09:51:03AM -0800, Andy Lutomirski wrote:
> > 
> >> Can we really not just change the lock asm to use 32-bit accesses for
> >> set_bit(), etc?  Sure, it will fail if the bit index is greater than
> >> 2^32, but that seems nuts.
> > 
> > There are 64bit architectures that do exactly that: Alpha, IA64.
> > 
> > And because of the byte 'optimization' from x86 we already could not
> > rely on word atomicity (we actually play games with multi-bit atomicity
> > for PG_waiters and clear_bit_unlock_is_negative_byte).
> 
> I read a couple pages of the paper you linked and I didn’t spot what
> you’re talking about as it refers to x86.  What are the relevant word
> properties of x86 bitops or the byte optimization?

The paper mostly deals with Power and ARM, x86 only gets sporadic
mention. It does present a way to reason about mixed size atomic
operations though.

And the bitops API is very much cross-architecture. And like I wrote in
that other email, having audited the atomic bitop width a number of
times now makes me say no to anything complicated.