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

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Message-ID: <ef45c2ca-942a-df83-22cf-690eaf761fb7@colorfullife.com>
Date:   Mon, 14 Oct 2019 19:49:56 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Waiman Long <longman@...hat.com>, 1vier1@....de,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 6/6] Documentation/memory-barriers.txt: Clarify cmpxchg()

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@...orfullife.com>
>> Cc: Waiman Long <longman@...hat.com>
>> Cc: Davidlohr Bueso <dave@...olabs.net>
>> Cc: Peter Zijlstra <peterz@...radead.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).



View attachment "0001-Update-Documentation-for-_-acquire-release-relaxed.patch" of type "text/x-patch" (2381 bytes)