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

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Message-ID: <63d9d6f6-05e8-473d-9d09-ce8d3a33ca39@paulmck-laptop>
Date: Tue, 30 Jan 2024 06:53:38 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: linux-kernel@...r.kernel.org,
	"E."@paulmck-thinkpad-p17-gen-1.smtp.subspace.kernel.org,
	McKenney <paulmck@...nel.org>
Cc: Alan Stern <stern@...land.harvard.edu>,
	Andrea Parri <parri.andrea@...il.com>, Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Nicholas Piggin <npiggin@...il.com>,
	David Howells <dhowells@...hat.com>,
	Jade Alglave <j.alglave@....ac.uk>,
	Luc Maranget <luc.maranget@...ia.fr>,
	Paul@...lmck-thinkpad-p17-gen-1.smtp.subspace.kernel.org,
	Akira Yokosawa <akiyks@...il.com>,
	Daniel Lustig <dlustig@...dia.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Mark Rutland <mark.rutland@....com>,
	Jonathan Corbet <corbet@....net>, linux-arch@...r.kernel.org,
	linux-doc@...r.kernel.org,
	Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: [PATCH doc] Emphasize that failed atomic operations give no ordering

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@...utronix.de>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Alan Stern <stern@...land.harvard.edu>
Cc: Andrea Parri <parri.andrea@...il.com>
Cc: Will Deacon <will@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: David Howells <dhowells@...hat.com>
Cc: Jade Alglave <j.alglave@....ac.uk>
Cc: Luc Maranget <luc.maranget@...ia.fr>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Akira Yokosawa <akiyks@...il.com>
Cc: Daniel Lustig <dlustig@...dia.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Jonathan Corbet <corbet@....net>
Cc: <linux-arch@...r.kernel.org>
Cc: <linux-doc@...r.kernel.org>

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()