From: Mathieu Desnoyers via lttng-dev <lttng-dev@lists.lttng.org>
To: "Ondřej Surý" <ondrej@sury.org>
Cc: lttng-dev@lists.lttng.org, paulmck <paulmck@kernel.org>
Subject: Re: [lttng-dev] [PATCH 7/7] Experiment: Add explicit memory barrier in free_completion()
Date: Tue, 21 Mar 2023 10:49:27 -0400 [thread overview]
Message-ID: <51974db5-f63f-7006-f2ae-37b3ca817041@efficios.com> (raw)
In-Reply-To: <1B64F3D0-EB5C-4023-BA15-37D0DBCBB1B3@sury.org>
On 2023-03-21 10:48, Ondřej Surý wrote:
>> 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.
Agreed, let's see how it holds up to testing under TSAN. :)
Thanks,
Mathieu
>
> Ondrej
> --
> Ondřej Surý (He/Him)
> ondrej@sury.org
>
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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next prev parent reply other threads:[~2023-03-21 14:49 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-17 21:37 [lttng-dev] [PATCH 0/7] Replace the custom code with gcc/clang __atomic builtins Ondřej Surý via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 1/7] Require __atomic builtins to build Ondřej Surý via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 2/7] Use gcc __atomic builtis for <urcu/uatomic.h> implementation Ondřej Surý via lttng-dev
2023-03-20 18:03 ` Mathieu Desnoyers via lttng-dev
2023-03-20 18:13 ` Mathieu Desnoyers via lttng-dev
2023-03-20 18:28 ` Ondřej Surý via lttng-dev
2023-03-20 18:38 ` Mathieu Desnoyers via lttng-dev
2023-03-20 18:41 ` Mathieu Desnoyers via lttng-dev
2023-03-20 19:38 ` Duncan Sands via lttng-dev
2023-03-21 20:26 ` Mathieu Desnoyers via lttng-dev
2023-03-22 8:24 ` Duncan Sands via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 3/7] Use __atomic_thread_fence() for cmm_barrier() Ondřej Surý via lttng-dev
2023-03-20 18:06 ` Mathieu Desnoyers via lttng-dev
2023-03-20 18:14 ` Mathieu Desnoyers via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 4/7] Replace the internal pointer manipulation with __atomic builtins Ondřej Surý via lttng-dev
2023-03-20 18:25 ` Mathieu Desnoyers via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 5/7] Use __atomic builtins to implement CMM_{LOAD, STORE}_SHARED Ondřej Surý via lttng-dev
2023-03-20 18:28 ` Mathieu Desnoyers via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 6/7] Fix: uatomic_or() need retyping to uintptr_t in rculfhash.c Ondřej Surý via lttng-dev
2023-03-20 18:31 ` Mathieu Desnoyers via lttng-dev
2023-03-21 10:15 ` Ondřej Surý via lttng-dev
2023-03-21 14:44 ` Mathieu Desnoyers via lttng-dev
2023-03-21 14:45 ` Mathieu Desnoyers via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 7/7] Experiment: Add explicit memory barrier in free_completion() Ondřej Surý via lttng-dev
2023-03-20 18:37 ` Mathieu Desnoyers via lttng-dev
2023-03-21 10:21 ` Ondřej Surý via lttng-dev
2023-03-21 14:46 ` Mathieu Desnoyers via lttng-dev
2023-03-21 14:48 ` Ondřej Surý via lttng-dev
2023-03-21 14:49 ` Mathieu Desnoyers via lttng-dev [this message]
2023-03-21 14:59 ` [lttng-dev] TSAN and the tests Ondřej Surý via lttng-dev
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