linux-kernel - Re: [PATCH v9 17/17] x86/split_lock: Warn on unaligned address in atomic bit operations

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Message-ID: <alpine.DEB.2.21.1906262352310.32342@nanos.tec.linutronix.de>
Date:   Thu, 27 Jun 2019 00:00:28 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Christopherson Sean J <sean.j.christopherson@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH v9 17/17] x86/split_lock: Warn on unaligned address in
 atomic bit operations

On Tue, 18 Jun 2019, Fenghua Yu wrote:

> An atomic bit operation operates one bit in a single unsigned long location
> in a bitmap. In 64-bit mode, the location is at:
> base address of the bitmap + (bit offset in the bitmap / 64) * 8
> 
> If the base address is unaligned to unsigned long, each unsigned long
> location operated by the atomic operation will be unaligned to unsigned
> long and a split lock issue will happen if the unsigned long location
> crosses two cache lines.

Stop harping on this split lock stuff.

Unalignedness is a problem per se as myself and others explained you a
gazillion times now.

The fact that it does not matter on x86 except when it crosses a cacheline
does not make it in any way a split lock issue.

The root cause is misalignment per se.

Aside of that this debug enhancement wants to be the first patch in the
series not the last.

Thanks,

	tglx