locking/percpu-rwsem: Replace waitqueue with rcuwait

The use of any kind of wait queue is an overkill for pcpu-rwsems.
While one option would be to use the less heavy simple (swait)
flavor, this is still too much for what pcpu-rwsems needs. For one,
we do not care about any sort of queuing in that the only (rare) time
writers (and readers, for that matter) are queued is when trying to
acquire the regular contended rw_sem. There cannot be any further
queuing as writers are serialized by the rw_sem in the first place.

Given that percpu_down_write() must not be called after exit_notify(),
we can replace the bulky waitqueue with rcuwait such that a writer
can wait for its turn to take the lock. As such, we can avoid the
queue handling and locking overhead.

Signed-off-by: Davidlohr Bueso <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Oleg Nesterov <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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