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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter
Date:   Thu, 21 Nov 2019 13:01:08 -0800
Message-Id: <066A48B7-296F-4953-89A6-588509FC0303@amacapital.net>
References: <20191121195634.GV4097@hirez.programming.kicks-ass.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>
In-Reply-To: <20191121195634.GV4097@hirez.programming.kicks-ass.net>
To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Nov 21, 2019, at 11:56 AM, Peter Zijlstra <peterz@infradead.org> wrote:=

>=20
> =EF=BB=BFOn Thu, Nov 21, 2019 at 09:51:03AM -0800, Andy Lutomirski wrote:
>=20
>> 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.
>=20
> There are 64bit architectures that do exactly that: Alpha, IA64.
>=20
> 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=E2=80=99t spot what=
 you=E2=80=99re talking about as it refers to x86.  What are the relevant wo=
rd properties of x86 bitops or the byte optimization?=