sched/completion: Serialize completion_done() with complete()

sched/completion: Serialize completion_done() with complete()

Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when
reading completion state" was not correct, without lock/unlock the code
like stop_machine_from_inactive_cpu()

	while (!completion_done())
		cpu_relax();

can return before complete() finishes its spin_unlock() which writes to
this memory. And spin_unlock_wait().

While at it, change try_wait_for_completion() to use READ_ONCE().

Reported-by: Paul E. McKenney <[email protected]>
Reported-by: Davidlohr Bueso <[email protected]>
Tested-by: Paul E. McKenney <[email protected]>
Signed-off-by: Oleg Nesterov <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
[ Added a comment with the barrier. ]
Cc: Linus Torvalds <[email protected]>
Cc: Nicholas Mc Guire <[email protected]>
Cc: [email protected]
Cc: [email protected]
Fixes: de30ec47302c ("sched/completion: Remove unnecessary ->wait.lock serialization when reading completion state")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>