On 3/19/2024 2:37 PM, Uladzislau Rezki wrote:
> On Tue, Mar 19, 2024 at 01:33:11PM -0400, Joel Fernandes wrote:
>>> On 3/19/2024 1:26 PM, Uladzislau Rezki wrote:
>>>>>>>>>>>> /*
>>>>>>>>>>>> @@ -1673,7 +1680,7 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
>>>>>>>>>>>> */
>>>>>>>>>>>> static void rcu_sr_normal_gp_cleanup(void)
>>>>>>>>>>>> {
>>>>>>>>>>>> - struct llist_node *wait_tail, *next, *rcu;
>>>>>>>>>>>> + struct llist_node *wait_tail, *next = NULL, *rcu = NULL;
>>>>>>>>>>>> int done = 0;
>>>>>>>>>>>>
>>>>>>>>>>>> wait_tail = rcu_state.srs_wait_tail;
>>>>>>>>>>>> @@ -1699,16 +1706,35 @@ static void rcu_sr_normal_gp_cleanup(void)
>>>>>>>>>>>> break;
>>>>>>>>>>>> }
>>>>>>>>>>>>
>>>>>>>>>>>> - // concurrent sr_normal_gp_cleanup work might observe this update.
>>>>>>>>>>>> - smp_store_release(&rcu_state.srs_done_tail, wait_tail);
>>>>>>>>>>>> + /*
>>>>>>>>>>>> + * Fast path, no more users to process. Remove the last wait head
>>>>>>>>>>>> + * if no inflight-workers. If there are in-flight workers, let them
>>>>>>>>>>>> + * remove the last wait head.
>>>>>>>>>>>> + */
>>>>>>>>>>>> + WARN_ON_ONCE(!rcu);
>>>>>>>>>>>>
>>>>>>>>>>> This assumption is not correct. An "rcu" can be NULL in fact.
>>>>>>>>>>
>>>>>>>>>> Hmm I could never trigger that. Are you saying that is true after Neeraj recent patch or something else?
>>>>>>>>>> Note, after Neeraj patch to handle the lack of heads availability, it could be true so I requested
>>>>>>>>>> him to rebase his patch on top of this one.
>>>>>>>>>>
>>>>>>>>>> However I will revisit my patch and look for if it could occur but please let me know if you knew of a sequence of events to make it NULL.
>>>>>>>>>>>
>>>>>>>>> I think we should agree on your patch first otherwise it becomes a bit
>>>>>>>>> messy or go with a Neeraj as first step and then work on youth. So, i
>>>>>>>>> reviewed this patch based on latest Paul's dev branch. I see that Neeraj
>>>>>>>>> needs further work.
>>>>>>>>
>>>>>>>> You are right. So the only change is to drop the warning and those braces. Agreed?
>>>>>>>>
>>>>>>> Let me check a bit. Looks like correct but just in case.
>>>>>>>
>>>>>>
>>>>>> Thanks. I was also considering improving it for the rcu == NULL case, as
>>>>>> below. I will test it more before re-sending.
>>>>>>
>>>>>> On top of my patch:
>>>>>>
>>>>>> ---8<-----------------------
>>>>>>
>>>>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>>>>>> index 0df659a878ee..a5ef844835d4 100644
>>>>>> --- a/kernel/rcu/tree.c
>>>>>> +++ b/kernel/rcu/tree.c
>>>>>> @@ -1706,15 +1706,18 @@ static void rcu_sr_normal_gp_cleanup(void)
>>>>>> break;
>>>>>> }
>>>>>>
>>>>>> +
>>>>>> + /* Last head stays. No more processing to do. */
>>>>>> + if (!rcu)
>>>>>> + return;
>>>>>> +
>>>>>
>>>>> Ugh, should be "if (!wait_head->next)" instead of "if (!rcu)". But
>>>>> in any case, the original patch except the warning should hold.
>>>>> Still, I am testing the above diff now.
>>>>>
>>>>> - Joel
>>>>>
>>>> Just in case, it is based on your patch:
>>>>
>>>> <snip>
>>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>>>> index bd29fe3c76bf..98546afe7c21 100644
>>>> --- a/kernel/rcu/tree.c
>>>> +++ b/kernel/rcu/tree.c
>>>> @@ -1711,29 +1711,25 @@ static void rcu_sr_normal_gp_cleanup(void)
>>>> * if no inflight-workers. If there are in-flight workers, let them
>>>> * remove the last wait head.
>>>> */
>>>> - WARN_ON_ONCE(!rcu);
>>>> - ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
>>>> -
>>>> - if (rcu && rcu_sr_is_wait_head(rcu) && rcu->next == NULL &&
>>>> - /* Order atomic access with list manipulation. */
>>>> - !atomic_read_acquire(&rcu_state.srs_cleanups_pending)) {
>>>> + if (wait_tail->next && rcu_sr_is_wait_head(wait_tail->next) && !wait_tail->next->next &&
>>>> + !atomic_read_acquire(&rcu_state.srs_cleanups_pending)) {
>>>
>>>
>>> Yes this also works. But also if wait_tail->next == NULL, then you do not need
>>> to queue worker for that case as well. I sent this as v3.
>>>
>> Sorry, I see you did add that later in the patch ;-). I think we have converged
>> on the final patch then, give or take the use of 'rcu' versus 'wait_tail->next'.
>>
> Just combine all parts into one place and resend :)
Yes sir ;)
- Joel
