Ordering / preemption of work in a workqueue preempt?

[Arun KS]

Hi Rajat,

On Fri, Nov 15, 2013 at 8:46 PM, Rajat Jain <rajatjain at juniper.net> wrote:
> Hi,
>
> I have a single work queue, on which I have scheduled a worker function [using queue_work(wq, fn)] in interrupt context.
>
> I get the interrupt twice before the work queue gets a chance to run, and hence the same function will get queued twice (with different private context - arguments etc) which is fine and expected.

You got it wrong here.

bool queue_work_on(int cpu, struct workqueue_struct *wq,
                   struct work_struct *work)
{
        bool ret = false;
        unsigned long flags;

        local_irq_save(flags);

        if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
                __queue_work(cpu, wq, work);
                ret = true;
        }

        local_irq_restore(flags);
        return ret;
}
EXPORT_SYMBOL(queue_work_on);

If you look at function queue_work_on, the function returns
immediately if WORK_STRUCT_PENDING_BIT was already set.
So in effect your second call to queue_work will just return.

HTH

Thanks,
Arun


>
> Questions:
>
> 1) Is it possible that the instance that was queued by 2nd interrupt, can get to run BEFORE the instance that was queued by 1st interrupt? In other words, is reordering possible?
>
> 2) Is it possible that one running instance of the function, can get preempted by second instance of the same work queue? I read through http://lwn.net/Articles/511421/ and it talks about same work queue cannot run on different CPU, but I have doubt about single CPU. If If I am writing a worker function, does my code have to be ready that it can be preempted by another instance of the same function?
>
> Please note that I understand that my worker function can preempted by other processes, my doubts are related to the same workqueue.
>
> Thanks,
>
> Rajat
>
>
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