Re: [patch 3/3] x86/process: Optimize TIF_NOTSC switch

[Thomas Gleixner]

On Thu, 15 Dec 2016, Andy Lutomirski wrote:
> On Thu, Dec 15, 2016 at 8:44 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > +static inline void cr4_toggle_bits(unsigned long mask)
> > +{
> > +       unsigned long cr4;
> > +
> > +       cr4 = this_cpu_read(cpu_tlbstate.cr4);
> > +       cr4 ^= mask;
> > +       this_cpu_write(cpu_tlbstate.cr4, cr4);
> > +       __write_cr4(cr4);
> > +}
> 
> This scares me for the same reason as BTF, although this should at
> least be less fragile.  But how about:

If that is fragile then all cr4 manipulation code is fragile because it
relies on cpu_tlbstate.cr4. The TIF flag and that per cpu thing are kept in
sync.

Thanks,

	tglx