Re: [lttng-dev] [PATCH 7/7] Experiment: Add explicit memory barrier in free_completion()

["Ondřej Surý via lttng-dev" <lttng-dev@lists.lttng.org>]

> On 21. 3. 2023, at 15:46, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
> On 2023-03-21 06:21, Ondřej Surý wrote:
>>> On 20. 3. 2023, at 19:37, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>>> 
>>> On 2023-03-17 17:37, Ondřej Surý via lttng-dev wrote:
>>>> FIXME: This is experiment that adds explicit memory barrier in the
>>>> free_completion in the workqueue.c, so ThreadSanitizer knows it's ok to
>>>> free the resources.
>>>> Signed-off-by: Ondřej Surý <ondrej@sury.org>
>>>> ---
>>>>  src/workqueue.c | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>> diff --git a/src/workqueue.c b/src/workqueue.c
>>>> index 1039d72..f21907f 100644
>>>> --- a/src/workqueue.c
>>>> +++ b/src/workqueue.c
>>>> @@ -377,6 +377,7 @@ void free_completion(struct urcu_ref *ref)
>>>>   struct urcu_workqueue_completion *completion;
>>>>     completion = caa_container_of(ref, struct urcu_workqueue_completion, ref);
>>>> + assert(!urcu_ref_get_unless_zero(&completion->ref));
>>> 
>>> Perhaps what we really want here is an ANNOTATE_UNPUBLISH_MEMORY_RANGE() of some sort ?
>> I guess?
>> My experience with TSAN tells me, that you need some kind of memory barrier when using acquire-release
>> semantics and you do:
>> if (__atomic_sub_fetch(obj->ref, __ATOMIC_RELEASE) == 0) {
>>   /* __ATOMIC_ACQUIRE needed here */
>>    free(obj);
>> }
>> we end up using following code in BIND 9:
>> if (__atomic_sub_fetch(obj->ref, __ATOMIC_ACQ_REL) == 0) {
>>    free(obj);
>> }
>> So, I am guessing after the change of uatomic_sub_return() to __ATOMIC_ACQ_REL,
>> this patch should no longer be needed.
> 
> Actually we want __ATOMIC_SEQ_CST, which is even stronger than ACQ_REL.

Yeah, I think I already did that, but wrote the email before that. Nevertheless, my main
point was that it should not be needed anymore.

Ondrej
--
Ondřej Surý (He/Him)
ondrej@sury.org

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