locking/mutexes: Delete the MUTEX_SHOW_NO_WAITER macro

MUTEX_SHOW_NO_WAITER() is a macro which checks for if there are
"no waiters" on a mutex by checking if the lock count is non-negative.
Based on feedback from the discussion in the earlier version of this
patchset, the macro is not very readable.

Furthermore, checking lock->count isn't always the correct way to
determine if there are "no waiters" on a mutex. For example, a negative
count on a mutex really only means that there "potentially" are
waiters. Likewise, there can be waiters on the mutex even if the count is
non-negative. Thus, "MUTEX_SHOW_NO_WAITER" doesn't always do what the name
of the macro suggests.

So this patch deletes the MUTEX_SHOW_NO_WAITERS() macro, directly
use atomic_read() instead of the macro, and adds comments which
elaborate on how the extra atomic_read() checks can help reduce
unnecessary xchg() operations.

Signed-off-by: Jason Low <jason.low2@hp.com>
Acked-by: Waiman Long <Waiman.Long@hp.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: akpm@linux-foundation.org
Cc: tim.c.chen@linux.intel.com
Cc: paulmck@linux.vnet.ibm.com
Cc: rostedt@goodmis.org
Cc: davidlohr@hp.com
Cc: scott.norton@hp.com
Cc: aswin@hp.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1402511843-4721-3-git-send-email-jason.low2@hp.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>