Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

Nicolas Pitre
2008-11-07T08:12:24Z

On Thu, 6 Nov 2008, Andrew Morton wrote:

> On Fri, 07 Nov 2008 00:23:44 -0500 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
>
> > #define cnt32_to_63(cnt_lo) \
> > ({ \
> > - static volatile u32 __m_cnt_hi; \
> > + static u32 __m_cnt_hi; \
> > union cnt32_to_63 __x; \
> > __x.hi = __m_cnt_hi; \
> > + smp_rmb(); /* read __m_cnt_hi before mmio cnt_lo */ \
> > __x.lo = (cnt_lo); \
> > if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
> > __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
>
> Oh dear. We have a macro which secretly maintains
> per-instantiation-site global state? And doesn't even implement locking
> to protect that state?

Please do me a favor and look for those very unfrequent posts I've sent
to lkml lately. I've explained it all at least 3 times so far, to Peter
Zijlstra, to David Howells, to Mathieu Desnoyers, and now to you.

> I mean, the darned thing is called from sched_clock(), which can be
> concurrently called on separate CPUs and which can be called from
> interrupt context (with an arbitrary nesting level!) while it was running
> in process context.

Yes! And this is so on *purpose*. Please take some time to read the
comment that goes along with it, and if you're still not convinced then
look for those explanation emails I've already posted.

> /*
> * Caller must provide locking to protect *caller_state
> */

NO! This is meant to be LOCK FREE!

Nicolas