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| author | Will Deacon <will.deacon@arm.com> | 2018-02-13 14:30:19 +0100 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2018-02-13 14:55:53 +0100 |
| commit | 61e02392d3c7ecac1f91c0a90a8043d67e081846 (patch) | |
| tree | 643df7dcf736a212690237f1b49ef108a2f096e6 | |
| parent | locking/qspinlock: Ensure node->count is updated before initialising node (diff) | |
| download | linux-61e02392d3c7ecac1f91c0a90a8043d67e081846.tar.xz linux-61e02392d3c7ecac1f91c0a90a8043d67e081846.zip | |
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>
| -rw-r--r-- | Documentation/atomic_bitops.txt | 7 | ||||
| -rw-r--r-- | 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) |