locking/atomic: scripts: Clarify ordering of conditional atomics

Conditional atomic operations (e.g. cmpxchg()) only provide ordering
when the condition holds; when the condition does not hold, the location
is not modified and relaxed ordering is provided. Where ordering is
needed for failed conditional atomics, it is necessary to use
smp_mb__before_atomic() and/or smp_mb__after_atomic().

This is explained tersely in memory-barriers.txt, and is implied but not
explicitly stated in the kerneldoc comments for the conditional
operations. The lack of an explicit statement has lead to some off-list
queries about the ordering semantics of failing conditional operations,
so evidently this is confusing.

Update the kerneldoc comments to explicitly describe the lack of ordering
for failed conditional atomic operations.

For most conditional atomic operations, this is written as:

  | If (${condition}), atomically updates @v to (${new}) with ${desc_order} ordering.
  | Otherwise, @v is not modified and relaxed ordering is provided.

For the try_cmpxchg() operations, this is written as:

  | If (${condition}), atomically updates @v to @new with ${desc_order} ordering.
  | Otherwise, @v is not modified, @old is updated to the current value of @v,
  | and relaxed ordering is provided.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Link: https://lore.kernel.org/r/20240209124010.2096198-1-mark.rutland@arm.com

1 files changed, 19 insertions, 5 deletions

diff --git a/include/linux/atomic/atomic-long.h b/include/linux/atomic/atomic-long.h
index c82947170ddc..3ef844b3ab8a 100644
--- a/include/linux/atomic/atomic-long.h
+++ b/include/linux/atomic/atomic-long.h
@@ -1352,6 +1352,7 @@ raw_atomic_long_xchg_relaxed(atomic_long_t *v, long new)
  * @new: long value to assign
  *
  * If (@v == @old), atomically updates @v to @new with full ordering.
+ * Otherwise, @v is not modified and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_cmpxchg() elsewhere.
  *
@@ -1374,6 +1375,7 @@ raw_atomic_long_cmpxchg(atomic_long_t *v, long old, long new)
  * @new: long value to assign
  *
  * If (@v == @old), atomically updates @v to @new with acquire ordering.
+ * Otherwise, @v is not modified and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_cmpxchg_acquire() elsewhere.
  *
@@ -1396,6 +1398,7 @@ raw_atomic_long_cmpxchg_acquire(atomic_long_t *v, long old, long new)
  * @new: long value to assign
  *
  * If (@v == @old), atomically updates @v to @new with release ordering.
+ * Otherwise, @v is not modified and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_cmpxchg_release() elsewhere.
  *
@@ -1418,6 +1421,7 @@ raw_atomic_long_cmpxchg_release(atomic_long_t *v, long old, long new)
  * @new: long value to assign
  *
  * If (@v == @old), atomically updates @v to @new with relaxed ordering.
+ * Otherwise, @v is not modified and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_cmpxchg_relaxed() elsewhere.
  *
@@ -1440,7 +1444,8 @@ raw_atomic_long_cmpxchg_relaxed(atomic_long_t *v, long old, long new)
  * @new: long value to assign
  *
  * If (@v == @old), atomically updates @v to @new with full ordering.
- * Otherwise, updates @old to the current value of @v.
+ * Otherwise, @v is not modified, @old is updated to the current value of @v,
+ * and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_try_cmpxchg() elsewhere.
  *
@@ -1463,7 +1468,8 @@ raw_atomic_long_try_cmpxchg(atomic_long_t *v, long *old, long new)
  * @new: long value to assign
  *
  * If (@v == @old), atomically updates @v to @new with acquire ordering.
- * Otherwise, updates @old to the current value of @v.
+ * Otherwise, @v is not modified, @old is updated to the current value of @v,
+ * and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_try_cmpxchg_acquire() elsewhere.
  *
@@ -1486,7 +1492,8 @@ raw_atomic_long_try_cmpxchg_acquire(atomic_long_t *v, long *old, long new)
  * @new: long value to assign
  *
  * If (@v == @old), atomically updates @v to @new with release ordering.
- * Otherwise, updates @old to the current value of @v.
+ * Otherwise, @v is not modified, @old is updated to the current value of @v,
+ * and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_try_cmpxchg_release() elsewhere.
  *
@@ -1509,7 +1516,8 @@ raw_atomic_long_try_cmpxchg_release(atomic_long_t *v, long *old, long new)
  * @new: long value to assign
  *
  * If (@v == @old), atomically updates @v to @new with relaxed ordering.
- * Otherwise, updates @old to the current value of @v.
+ * Otherwise, @v is not modified, @old is updated to the current value of @v,
+ * and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_try_cmpxchg_relaxed() elsewhere.
  *
@@ -1677,6 +1685,7 @@ raw_atomic_long_add_negative_relaxed(long i, atomic_long_t *v)
  * @u: long value to compare with
  *
  * If (@v != @u), atomically updates @v to (@v + @a) with full ordering.
+ * Otherwise, @v is not modified and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_fetch_add_unless() elsewhere.
  *
@@ -1699,6 +1708,7 @@ raw_atomic_long_fetch_add_unless(atomic_long_t *v, long a, long u)
  * @u: long value to compare with
  *
  * If (@v != @u), atomically updates @v to (@v + @a) with full ordering.
+ * Otherwise, @v is not modified and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_add_unless() elsewhere.
  *
@@ -1719,6 +1729,7 @@ raw_atomic_long_add_unless(atomic_long_t *v, long a, long u)
  * @v: pointer to atomic_long_t
  *
  * If (@v != 0), atomically updates @v to (@v + 1) with full ordering.
+ * Otherwise, @v is not modified and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_inc_not_zero() elsewhere.
  *
@@ -1739,6 +1750,7 @@ raw_atomic_long_inc_not_zero(atomic_long_t *v)
  * @v: pointer to atomic_long_t
  *
  * If (@v >= 0), atomically updates @v to (@v + 1) with full ordering.
+ * Otherwise, @v is not modified and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_inc_unless_negative() elsewhere.
  *
@@ -1759,6 +1771,7 @@ raw_atomic_long_inc_unless_negative(atomic_long_t *v)
  * @v: pointer to atomic_long_t
  *
  * If (@v <= 0), atomically updates @v to (@v - 1) with full ordering.
+ * Otherwise, @v is not modified and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_dec_unless_positive() elsewhere.
  *
@@ -1779,6 +1792,7 @@ raw_atomic_long_dec_unless_positive(atomic_long_t *v)
  * @v: pointer to atomic_long_t
  *
  * If (@v > 0), atomically updates @v to (@v - 1) with full ordering.
+ * Otherwise, @v is not modified and relaxed ordering is provided.
  *
  * Safe to use in noinstr code; prefer atomic_long_dec_if_positive() elsewhere.
  *
@@ -1795,4 +1809,4 @@ raw_atomic_long_dec_if_positive(atomic_long_t *v)
 }
 
 #endif /* _LINUX_ATOMIC_LONG_H */
-// 4ef23f98c73cff96d239896175fd26b10b88899e
+// 1c4a26fc77f345342953770ebe3c4d08e7ce2f9a