On Thu, 6 Nov 2008, Andrew Morton wrote:
> On Fri, 07 Nov 2008 00:23:44 -0500 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> 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
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