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Subject[tip:locking/urgent] locking/atomic/bitops: Document and clarify ordering semantics for failed test_and_{}_bit()
Commit-ID:  61e02392d3c7ecac1f91c0a90a8043d67e081846
Gitweb: https://git.kernel.org/tip/61e02392d3c7ecac1f91c0a90a8043d67e081846
Author: Will Deacon <will.deacon@arm.com>
AuthorDate: Tue, 13 Feb 2018 13:30:19 +0000
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 13 Feb 2018 14:55:53 +0100

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 | 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 5550bfdc..be70b32 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 bc39757..67ab280 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)
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