atomic/bitops: Clarify ordering semantics for failed test_and_{}_bit()

Message ID 1518528619-20049-1-git-send-email-will.deacon@arm.com
State Accepted
Commit 61e02392d3c7ecac1f91c0a90a8043d67e081846
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Series
  • atomic/bitops: Clarify ordering semantics for failed test_and_{}_bit()
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Commit Message

Will Deacon Feb. 13, 2018, 1:30 p.m.
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@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

---
 Documentation/atomic_bitops.txt   | 7 ++++++-
 include/asm-generic/bitops/lock.h | 3 ++-
 2 files changed, 8 insertions(+), 2 deletions(-)

-- 
2.1.4

Comments

Peter Zijlstra Feb. 13, 2018, 1:51 p.m. | #1
On Tue, Feb 13, 2018 at 01:30:19PM +0000, Will Deacon wrote:
> 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.


You also looked at a bunch of users, right? And while you found some
dodgy ones, they were not more broken because of this IIRC.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>


> Cc: Peter Zijlstra <peterz@infradead.org>

> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

> Signed-off-by: Will Deacon <will.deacon@arm.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

>

Patch

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)