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SubjectRe: [PATCH 6/8] RCU, workqueue: Implement rcu_work
On Tue, Mar 20, 2018 at 12:45 AM, Tejun Heo <tj@kernel.org> wrote:
> Hello, Lai.
>
> On Fri, Mar 16, 2018 at 02:01:35PM +0800, Lai Jiangshan wrote:
>> > +bool flush_rcu_work(struct rcu_work *rwork)
>> > +{
>> > + if (test_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&rwork->work))) {
>> > + rcu_barrier();
>> > + flush_work(&rwork->work);
>> > + return true;
>>
>> A possible tiny improvement: check if it was already queued on wq.
>> For example:
>>
>> if (test_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&rwork->work))) {
>> if (!flush_work(&rwork->work)) {
>> rcu_barrier();
>> flush_work(&rwork->work);
>> }
>> return true;
>
> But this breaks the guarantee that flush_work waits for the latest
> queueing instance. Please consider the following scenario.

Oh, I'm sorry I was wrong. It is so evident that
"flush_work(&rwork->work) return true"
doesn't equals to "it has been queued on wq".

To detect if "it has been queued on wq" requires
a bunch of code. It is not worthy to complicate
this function.

>
>
> 1. rcu-work is queued
> 2. rcu-work starts executing
> 3. rcu-work is queued again
> 4. rcu-work is flushed
> 5. execution finishes
> 6. flush finishes
> 7. execution finishes
>
> 6 should happen after 7 but it didn't.
>
> Thanks.
>
> --
> tejun

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