Re: [PATCH v2] clarify usage expectations for cnt32_to_63()

Nicolas Pitre
2008-11-10T23:15:56Z

On Mon, 10 Nov 2008, Andrew Morton wrote:

> On Mon, 10 Nov 2008 16:34:54 -0500 (EST)
> Nicolas Pitre <nico@cam.org> wrote:
>
> > > It is far better to make the management of the state explicit and at
> > > the control of the caller. Get the caller to allocate the state and
> > > pass its address into this function. Simple, clear, explicit and
> > > robust.
> >
> > Sigh... What about this compromize then?
> >
> > diff --git a/include/linux/cnt32_to_63.h b/include/linux/cnt32_to_63.h
> > index 7605fdd..74ce767 100644
> > --- a/include/linux/cnt32_to_63.h
> > +++ b/include/linux/cnt32_to_63.h
> > @@ -32,8 +32,9 @@ union cnt32_to_63 {
> >
> >
> > /**
> > - * cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
> > + * __cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
> > * @cnt_lo: The low part of the counter
> > + * @cnt_hi_p: Pointer to storage for the extended part of the counter
> > *
> > * Many hardware clock counters are only 32 bits wide and therefore have
> > * a relatively short period making wrap-arounds rather frequent. This
> > @@ -75,16 +76,31 @@ union cnt32_to_63 {
> > * clear-bit instruction. Otherwise caller must remember to clear the top
> > * bit explicitly.
> > */
> > -#define cnt32_to_63(cnt_lo) \
> > +#define __cnt32_to_63(cnt_lo, cnt_hi_p) \
> > ({ \
> > - static u32 __m_cnt_hi; \
> > union cnt32_to_63 __x; \
> > - __x.hi = __m_cnt_hi; \
> > + __x.hi = *(cnt_hi_p); \
> > smp_rmb(); \
> > __x.lo = (cnt_lo); \
> > if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
> > - __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
> > + *(cnt_hi_p) = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
> > __x.val; \
> > })
>
> This references its second argument twice, which can cause correctness
> or efficiency problems.
>
> There is no reason that this had to be implemented in cpp.
> Implementing it in C will fix the above problem.

No, it won't, for correctness and efficiency reasons.

And I've explained why already.

No need to discuss this further if you can't get it.

Nicolas