locking/xchg/alpha: Fix xchg() and cmpxchg() memory ordering bugs

locking/xchg/alpha: Fix xchg() and cmpxchg() memory ordering bugs

Successful RMW operations are supposed to be fully ordered, but
Alpha's xchg() and cmpxchg() do not meet this requirement.

Will Deacon noticed the bug:

  > So MP using xchg:
  >
  > WRITE_ONCE(x, 1)
  > xchg(y, 1)
  >
  > smp_load_acquire(y) == 1
  > READ_ONCE(x) == 0
  >
  > would be allowed.

... which thus violates the above requirement.

Fix it by adding a leading smp_mb() to the xchg() and cmpxchg() implementations.

Reported-by: Will Deacon <[email protected]>
Signed-off-by: Andrea Parri <[email protected]>
Acked-by: Paul E. McKenney <[email protected]>
Cc: Alan Stern <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Ivan Kokshaysky <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Richard Henderson <[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]>