The documentation in memory-barriers.txt claims that
smp_mb__{before,after}_atomic() are for atomic ops that do not return a
value.
This is misleading and doesn't match the example in atomic_t.txt,
and e.g. smp_mb__before_atomic() may and is used together with
cmpxchg_relaxed() in the wake_q code.
The purpose of e.g. smp_mb__before_atomic() is to "upgrade" a following
RMW atomic operation to a full memory barrier.
The return code of the atomic operation has no impact, so all of the
following examples are valid:
1)
smp_mb__before_atomic();
atomic_add();
2)
smp_mb__before_atomic();
atomic_xchg_relaxed();
3)
smp_mb__before_atomic();
atomic_fetch_add_relaxed();
Invalid would be:
smp_mb__before_atomic();
atomic_set();
Signed-off-by: Manfred Spraul <manf...@colorfullife.com>
Cc: Waiman Long <long...@redhat.com>
Cc: Davidlohr Bueso <d...@stgolabs.net>
Cc: Peter Zijlstra <pet...@infradead.org>
---
Documentation/memory-barriers.txt | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/Documentation/memory-barriers.txt
b/Documentation/memory-barriers.txt
index 1adbb8a371c7..52076b057400 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1873,12 +1873,13 @@ There are some more advanced barrier functions:
(*) smp_mb__before_atomic();
(*) smp_mb__after_atomic();
- These are for use with atomic (such as add, subtract, increment and
- decrement) functions that don't return a value, especially when used for
- reference counting. These functions do not imply memory barriers.
+ These are for use with atomic RMW functions (such as add, subtract,
+ increment, decrement, failed conditional operations, ...) that do
+ not imply memory barriers, but where the code needs a memory barrier,
+ for example when used for reference counting.
- These are also used for atomic bitop functions that do not return a
- value (such as set_bit and clear_bit).
+ These are also used for atomic RMW bitop functions that do imply a full
+ memory barrier (such as set_bit and clear_bit).
As an example, consider a piece of code that marks an object as being dead
and then decrements the object's reference count:
--
2.21.0
