[PATCH doc] Emphasize that failed atomic operations give no ordering

[Paul E. McKenney]

The ORDERING section of Documentation/atomic_t.txt can easily be read as
saying that conditional atomic RMW operations that fail are ordered when
those operations have the _acquire() or _release() prefixes.  This is
not the case, therefore update this section to make it clear that failed
conditional atomic RMW operations provide no ordering.

Reported-by: Anna-Maria Behnsen <anna-maria@xxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Jade Alglave <j.alglave@xxxxxxxxx>
Cc: Luc Maranget <luc.maranget@xxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Akira Yokosawa <akiyks@xxxxxxxxx>
Cc: Daniel Lustig <dlustig@xxxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: <linux-arch@xxxxxxxxxxxxxxx>
Cc: <linux-doc@xxxxxxxxxxxxxxx>

diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index d7adc6d543db4..bee3b1bca9a7b 100644
--- a/Documentation/atomic_t.txt
+++ b/Documentation/atomic_t.txt
@@ -171,14 +171,14 @@ The rule of thumb:
  - RMW operations that are conditional are unordered on FAILURE,
    otherwise the above rules apply.
 
-Except of course when an operation has an explicit ordering like:
+Except of course when a successful operation has an explicit ordering like:
 
  {}_relaxed: unordered
  {}_acquire: the R of the RMW (or atomic_read) is an ACQUIRE
  {}_release: the W of the RMW (or atomic_set)  is a  RELEASE
 
 Where 'unordered' is against other memory locations. Address dependencies are
-not defeated.
+not defeated.  Conditional operations are still unordered on FAILURE.
 
 Fully ordered primitives are ordered against everything prior and everything
 subsequent. Therefore a fully ordered primitive is like having an smp_mb()