linux-kernel - Re: [PATCH v2 1/3] x86/cpufeatures: Enumerate user wait instructions

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Date:   Thu, 21 Feb 2019 10:22:13 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Tao Xu <tao3.xu@...el.com>, jingqi.liu@...el.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        H Peter Anvin <hpa@...or.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v2 1/3] x86/cpufeatures: Enumerate user wait instructions

On Wed, Feb 20, 2019 at 10:37:27PM -0800, Andy Lutomirski wrote:
> On Wed, Feb 20, 2019 at 7:44 PM Tao Xu <tao3.xu@...el.com> wrote:

> > +static ssize_t umwait_enable_c0_2_show(struct device *dev,
> > +                                      struct device_attribute *attr,
> > +                                      char *buf)
> > +{
> > +       return sprintf(buf, "%d\n", umwait_enable_c0_2);
> 
> I realize that it's traditional to totally ignore races in sysfs and
> such, but it's a bad tradition.  Please either READ_ONCE it with a
> comment or take the mutex.

Note that when using READ_ONCE(), the other side must use WRITE_ONCE().
Mixed usage is not valid.