Improve a few details in perfcounter call-chain recording that
makes use of fast-GUP:
- Use ACCESS_ONCE() to observe the pte value. ptes are fundamentally
racy and can be changed on another CPU, so we have to be careful
about how we access them. The PAE branch is already careful with
read-barriers - but the non-PAE and 64-bit side needs an
ACCESS_ONCE() to make sure the pte value is observed only once.
- make the checks a bit stricter so that we can feed it any kind of
cra^H^H^H user-space input ;-)
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
0c87197142