ipc/mqueue: Implement lockless pipelined wakeups

This patch moves the wakeup_process() invocation so it is not done under
the info->lock by making use of a lockless wake_q. With this change, the
waiter is woken up once it is STATE_READY and it does not need to loop
on SMP if it is still in STATE_PENDING. In the timeout case we still need
to grab the info->lock to verify the state.

This change should also avoid the introduction of preempt_disable() in -rt
which avoids a busy-loop which pools for the STATE_PENDING -> STATE_READY
change if the waiter has a higher priority compared to the waker.

Additionally, this patch micro-optimizes wq_sleep by using the cheaper
cousin of set_current_state(TASK_INTERRUPTABLE) as we will block no
matter what, thus get rid of the implied barrier.

Signed-off-by: Davidlohr Bueso <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: George Spelvin <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Manfred Spraul <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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