On Wed, May 2, 2012 at 2:54 PM, Paul E. McKenney
[off-list ref] wrote:

On Thu, May 03, 2012 at 07:20:15AM +1000, Benjamin Herrenschmidt wrote:

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On Wed, 2012-05-02 at 13:25 -0700, Hugh Dickins wrote:

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Got it at last. =C2=A0Embarrassingly obvious. =C2=A0__rcu_read_lock() =

and

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__rcu_read_unlock() are not safe to be using __this_cpu operations,
the cpu may change in between the rmw's read and write: they should
be using this_cpu operations (or, I put preempt_disable/enable in the
__rcu_read_unlock below). =C2=A0__this_cpus there work out fine on x86=

,

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which was given good instructions to use; but not so well on PowerPC.

I've been running successfully for an hour now with the patch below;
but I expect you'll want to consider the tradeoffs, and may choose a
different solution.

Didn't Linus recently rant about these __this_cpu vs this_cpu nonsense ?

I thought that was going out..

Linus did rant about __raw_get_cpu_var() because it cannot use the x86
%fs segement overrides a bit more than a month ago. =C2=A0The __this_cpu
stuff is useful if you have preemption disabled -- avoids the extra
layer of preempt_disable().

Or was this a different rant?

https://lkml.org/lkml/2011/11/29/321

I think it ended up with Christoph removing the more egregious
variants, but this_cpu_that and __this_cpu_the_other remaining.

Hugh