locking/qspinlock: Add pending bit

Because the qspinlock needs to touch a second cacheline (the per-cpu
mcs_nodes[]); add a pending bit and allow a single in-word spinner
before we punt to the second cacheline.

It is possible so observe the pending bit without the locked bit when
the last owner has just released but the pending owner has not yet
taken ownership.

In this case we would normally queue -- because the pending bit is
already taken. However, in this case the pending bit is guaranteed
to be released 'soon', therefore wait for it and avoid queueing.

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Waiman Long <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Daniel J Blueman <[email protected]>
Cc: David Vrabel <[email protected]>
Cc: Douglas Hatch <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Raghavendra K T <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Scott J Norton <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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