Re: [Xen-devel] [PATCH v11 1/5] xen: move xen_setup_runstate_info and get_runstate_snapshot to drivers/xen/time.c

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 5 Nov 2015, Mark Rutland wrote:
> Hi,
> 
> > +static u64 get64(const u64 *p)
> > +{
> > +   u64 ret;
> > +
> > +   if (BITS_PER_LONG < 64) {
> > +           u32 *p32 = (u32 *)p;
> > +           u32 h, l;
> > +
> > +           /*
> > +            * Read high then low, and then make sure high is
> > +            * still the same; this will only loop if low wraps
> > +            * and carries into high.
> > +            * XXX some clean way to make this endian-proof?
> > +            */
> > +           do {
> > +                   h = p32[1];
> > +                   barrier();
> > +                   l = p32[0];
> > +                   barrier();
> > +           } while (p32[1] != h);
> 
> I realise this is simply a move of existing code, but it may be better
> to instead have:
> 
> do {
>       h = READ_ONCE(p32[1]);
>       l = READ_ONCE(p32[0]);
> } while (READ_ONCE(p32[1] != h);
> 
> Which ensures that each load is a single access (though it almost
> certainly would be anyway), and prevents the compiler from having to
> reload any other memory locations (which the current barrier() usage
> forces).

I am happy to make these changes, however for code clarity and review
simplicity I'll keep them on a separate patch (I like code movement to
remain code movement). I can squash the two patches together when
committing, if necessary.


> > +
> > +           ret = (((u64)h) << 32) | l;
> > +   } else
> > +           ret = *p;
> 
> Likewise, this would be better as READ_ONCE(*p), to force a single
> access.
> 
> > +
> > +   return ret;
> > +}
> 
> > +   do {
> > +           state_time = get64(&state->state_entry_time);
> > +           barrier();
> > +           *res = *state;
> > +           barrier();
> 
> You can also have:
> 
>       *res = READ_ONCE(*state);
> 
> That will which will handle the barriers implicitly.
> 
> Thanks,
> Mark.
> 
> > +   } while (get64(&state->state_entry_time) != state_time);
> > +}
> 

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