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authorWill Deacon <will.deacon@arm.com>2018-02-13 13:30:19 +0000
committerIngo Molnar <mingo@kernel.org>2018-02-13 14:55:53 +0100
commit61e02392d3c7ecac1f91c0a90a8043d67e081846 (patch)
tree643df7dcf736a212690237f1b49ef108a2f096e6 /Documentation
parent11dc13224c975efcec96647a4768a6f1bb7a19a8 (diff)
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>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/atomic_bitops.txt7
1 files changed, 6 insertions, 1 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