Re: [PATCH 6/6] Documentation/memory-barriers.txt: Clarify cmpxchg()

[Manfred Spraul <manfred@colorfullife.com>]

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Hello Peter,

On 10/14/19 3:03 PM, Peter Zijlstra wrote:
> On Sat, Oct 12, 2019 at 07:49:58AM +0200, Manfred Spraul wrote:
>> 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:
> The value return of atomic ops is relevant in so far that
> (traditionally) all value returning atomic ops already implied full
> barriers. That of course changed when we added
> _release/_acquire/_relaxed variants.
I've updated the Change description accordingly
>> 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 <manfred@colorfullife.com>
>> Cc: Waiman Long <longman@redhat.com>
>> Cc: Davidlohr Bueso <dave@stgolabs.net>
>> Cc: Peter Zijlstra <peterz@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
> s/do/do not/ ?
Sorry, yes, of course
>> +     memory barrier (such as set_bit and clear_bit).



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From 61c85a56994e32ea393af9debef4cccd9cd24abd Mon Sep 17 00:00:00 2001
From: Manfred Spraul <manfred@colorfullife.com>
Date: Fri, 11 Oct 2019 10:33:26 +0200
Subject: [PATCH] Update Documentation for _{acquire|release|relaxed}()

When adding the _{acquire|release|relaxed}() variants of some atomic
operations, it was forgotten to update Documentation/memory_barrier.txt:

smp_mb__{before,after}_atomic() is now indended for all RMW operations
that do not imply a full memory barrier.

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();

Fixes: 654672d4ba1a ("locking/atomics: Add _{acquire|release|relaxed}() variants of some atomic operations")

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
---
 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..08090eea3751 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 not 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