sched/wake_q: Document wake_q_add()

The only guarantee provided by wake_q_add() is that a wakeup will
happen after it, it does _NOT_ guarantee the wakeup will be delayed
until the matching wake_up_q().

If wake_q_add() fails the cmpxchg() a concurrent wakeup is pending and
that can happen at any time after the cmpxchg(). This means we should
not rely on the wakeup happening at wake_q_up(), but should be ready
for wake_q_add() to issue the wakeup.

The delay; if provided (most likely); should only result in more efficient
behaviour.

Reported-by: Yongji Xie <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Waiman Long <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>

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