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Date:   Thu, 21 Nov 2019 20:56:34 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     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
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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).

Also, there's a fun paper on the properties of mixed size atomic
operations for when you want to hurt your brain real bad:

  https://www.cl.cam.ac.uk/~pes20/popl17/mixed-size.pdf

_If_ we're going to change the bitops interface, I would propose we
change it to u32 and mandate every operation is indeed 32bit wide.