context_tracking: Protect against recursion

Context tracking recursion can happen when an exception triggers
in the middle of a call to a context tracking probe.

This special case can be caused by vmalloc faults. If an access
to a memory area allocated by vmalloc happens in the middle of
context_tracking_enter(), we may run into an endless fault loop
because the exception in turn calls context_tracking_enter()
which faults on the same vmalloc'ed memory, triggering an
exception again, etc...

Some rare crashes have been reported so lets protect against
this with a recursion counter.

Reported-by: Dave Jones <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Rafael J . Wysocki <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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