locking/atomic/bitops: Document and clarify ordering semantics for failed test_and_{}_bit()

A test_and_{}_bit() operation fails if the value of the bit is such that
the modification does not take place. For example, if test_and_set_bit()
returns 1. In these cases, follow the behaviour of cmpxchg and allow the
operation to be unordered. This also applies to test_and_set_bit_lock()
if the lock is found to be be taken already.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1518528619-20049-1-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Documentation/atomic_bitops.txt

@@ -58,7 +58,12 @@ Like with atomic_t, the rule of thumb is:
 
  - RMW operations that have a return value are fully ordered.
 
-Except for test_and_set_bit_lock() which has ACQUIRE semantics and
+ - RMW operations that are conditional are unordered on FAILURE,
+   otherwise the above rules apply. In the case of test_and_{}_bit() operations,
+   if the bit in memory is unchanged by the operation then it is deemed to have
+   failed.
+
+Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and
 clear_bit_unlock() which has RELEASE semantics.
 
 Since a platform only has a single means of achieving atomic operations

include/asm-generic/bitops/lock.h

@@ -7,7 +7,8 @@
  * @nr: Bit to set
  * @addr: Address to count from
  *
- * This operation is atomic and provides acquire barrier semantics.
+ * This operation is atomic and provides acquire barrier semantics if
+ * the returned value is 0.
  * It can be used to implement bit locks.
  */
 #define test_and_set_bit_lock(nr, addr)	test_and_set_bit(nr, addr)