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

Web:        https://git.kernel.org/torvalds/c/61e02392d3c7ecac1f91c0a90a8043d67e081846
Commit:     61e02392d3c7ecac1f91c0a90a8043d67e081846
Parent:     11dc13224c975efcec96647a4768a6f1bb7a19a8
Refname:    refs/heads/master
Author:     Will Deacon <[email protected]>
AuthorDate: Tue Feb 13 13:30:19 2018 +0000
Committer:  Ingo Molnar <[email protected]>
CommitDate: Tue Feb 13 14:55:53 2018 +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 <[email protected]>
    Acked-by: Peter Zijlstra (Intel) <[email protected]>
    Cc: Linus Torvalds <[email protected]>
    Cc: Paul E. McKenney <[email protected]>
    Cc: Thomas Gleixner <[email protected]>
    Link: http://lkml.kernel.org/r/[email protected]
    Signed-off-by: Ingo Molnar <[email protected]>
---
 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)
--
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