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

[Peter Zijlstra]

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@xxxxxxxxx> 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.