On Thu, Jun 09, 2016 at 02:26:59PM +0200, Paolo Bonzini wrote:
>
>
> On 09/06/2016 14:16, Peter Zijlstra wrote:
> > On Sat, May 28, 2016 at 08:27:43PM +0800, Minfei Huang wrote:
> >> +++ b/arch/x86/kernel/pvclock.c
> >> @@ -61,11 +61,14 @@ void pvclock_resume(void)
> >> u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src)
> >> {
> >> unsigned version;
> >> - cycle_t ret;
> >> u8 flags;
> >>
> >> do {
> >> - version = __pvclock_read_cycles(src, &ret, &flags);
> >> + version = src->version;
> >> + /* Make the latest version visible */
> >> + smp_rmb();
> >> +
> >> + flags = src->flags;
> >
> > Using a seqcount to load a single byte is insane ;-)
>
> Only if you know that the writer will not write that byte twice within a
> critical section...
>
> Which I guess we do know in this case because the write side is just a
> memcpy, but it's still a bit safer when it's not specified by the
> pvclock API. It's not a fast path anyway, it runs literally twice at
> startup.
Fair enough; just thought it was really silly code.
