linux-kernel - Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter

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Message-ID: <20191122203105.GE2844@hirez.programming.kicks-ass.net>
Date:   Fri, 22 Nov 2019 21:31:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        David Laight <David.Laight@...lab.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by
 kernel parameter

On Fri, Nov 22, 2019 at 05:48:14PM +0000, Luck, Tony wrote:
> > When we use byte ops, we must consider the word as 4 independent
> > variables. And in that case the later load might observe the lock-byte
> > state from 3, because the modification to the lock byte from 4 is in
> > CPU2's store-buffer.
> 
> So we absolutely violate this with the optimization for constant arguments
> to set_bit(), clear_bit() and change_bit() that are implemented as byte ops.
> 
> So is code that does:
> 
> 	set_bit(0, bitmap);
> 
> on one CPU. While another is doing:
> 
> 	set_bit(mybit, bitmap);
> 
> on another CPU safe? The first operates on just one byte, the second  on 8 bytes.

It is safe if all you care about is the consistency of that one bit.