linux-kernel - [PATCH] atomic/bitops: Clarify ordering semantics for failed test_and_{}

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Message-Id: <1518528619-20049-1-git-send-email-will.deacon@arm.com>
Date:   Tue, 13 Feb 2018 13:30:19 +0000
From:   Will Deacon <will.deacon@....com>
To:     linux-kernel@...r.kernel.org
Cc:     Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH] atomic/bitops: 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.

Cc: Peter Zijlstra <peterz@...radead.org>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Will Deacon <will.deacon@....com>
---
 Documentation/atomic_bitops.txt   | 7 ++++++-
 include/asm-generic/bitops/lock.h | 3 ++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/Documentation/atomic_bitops.txt b/Documentation/atomic_bitops.txt
index 5550bfdcce5f..be70b32c95d9 100644
--- a/Documentation/atomic_bitops.txt
+++ b/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
diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h
index bc397573c43a..67ab280ad134 100644
--- a/include/asm-generic/bitops/lock.h
+++ b/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)
-- 
2.1.4