ack to cpu IRQ level 14 thus letting NMIs back in and
we recurse.

The only path which that does that in the perf event IRQ
handling path is the code supporting frequency based events.  It
uses cpu_clock().

cpu_clock() simply invokes sched_clock() with IRQs disabled.

And that's a fundamental bug all on it's own, particularly for
the HAVE_UNSTABLE_SCHED_CLOCK case.  NMIs can thus get into the
sched_clock() code interrupting the local IRQ disable code
sections of it.

Furthermore, for the not-HAVE_UNSTABLE_SCHED_CLOCK case, the IRQ
disabling done by cpu_clock() is just pure overhead and
completely unnecessary.

So the core problem is that sched_clock() is not NMI safe, but
we are invoking it from NMI contexts in the perf events code
(via cpu_clock()).

A less important issue is the overhead of IRQ disabling when it
isn't necessary in cpu_clock().

CONFIG_HAVE_UNSTABLE_SCHED_CLOCK architectures are not
affected by this patch.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <20091213.182502.215092085.davem@davemloft.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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