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

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Date:   Thu, 21 Nov 2019 09:51:03 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Tony Luck <tony.luck@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by
 kernel parameter

On Thu, Nov 21, 2019 at 9:43 AM David Laight <David.Laight@...lab.com> wrote:
>
> From: Ingo Molnar
> > Sent: 21 November 2019 17:12
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> ...
> > > This feature MUST be default enabled, otherwise everything will
> > > be/remain broken and we'll end up in the situation where you can't use
> > > it even if you wanted to.
> >
> > Agreed.
>
> Before it can be enabled by default someone needs to go through the
> kernel and fix all the code that abuses the 'bit' functions by using them
> on int[] instead of long[].
>
> I've only seen one fix go through for one use case of one piece of code
> that repeatedly uses potentially misaligned int[] arrays for bitmasks.
>

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.

(Why the *hell* do the bitops use long anyway?  They're *bit masks*
for crying out loud.  As in, users generally want to operate on fixed
numbers of bits.)

--Andy