wait: introduce wait_event_exclusive_cmd

wait: introduce wait_event_exclusive_cmd

It's just a variant of wait_event_cmd(), with exclusive flag being set.

For cases like RAID5, which puts many processes to sleep until 1/4
resources are free, a wake_up wakes up all processes to run, but
there is one process being able to get the resource as it's protected
by a spin lock. That ends up introducing heavy lock contentions, and
hurts performance badly.

Here introduce wait_event_exclusive_cmd to relieve the lock contention
naturally by letting wake_up just wake up one process.

Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
v2: its assumed that wait*() and __wait*() have the same arguments - peterz

Acked-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Yuanhan Liu <[email protected]>
Signed-off-by: NeilBrown <[email protected]>