Re: [PATCH 0/2] jump label: 2.6.38 updates

[Peter Zijlstra]

On Mon, 2011-02-14 at 17:50 +0000, Will Newton wrote:
> On Mon, Feb 14, 2011 at 5:43 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Mon, 2011-02-14 at 17:38 +0000, Will Newton wrote:
> >> On Mon, Feb 14, 2011 at 5:27 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >>
> >> >> So all but a few have basically (as you said on IRC)
> >> >> #define atomic_read(v) ACCESS_ONCE(v)
> >> >
> >> > ACCESS_ONCE(v->counter), but yeah :-)
> >>
> >> I maintain an out-of-tree architecture where that isn't the case
> >> unfortunately [1]. Not expecting any special favours for being
> >> out-of-tree of course, but just thought I would add that data point.
> >>
> >> [1] Our atomic operations go around the cache rather than through it,
> >> so the value of an atomic cannot be read with a normal load
> >> instruction.
> >
> > Cannot how? It would observe a stale value? That is acceptable for
> > atomic_read().
> 
> It would observe a stale value, but that value would only be updated
> when the cache line was reloaded from main memory which would have to
> be triggered by either eviction or cache flushing. So it could get
> pretty stale. Whilst that's probably within the spec. of atomic_read I
> suspect it would lead to problems in practice. I could be wrong
> though.

Arguable, finding such cases would be a Good (TM) thing.. but yeah, I
can imagine you're not too keen on being the one finding them.

Luckily it looks like you're in the same boat as blackfin-smp is.

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