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    SubjectRe: [patch, minor] workqueue: consistently use 'err' in __create_workqueue_key()
    On 07/29, Dmitry Adamushko wrote:
    >
    > 2008/7/29 Oleg Nesterov <oleg@tv-sign.ru>:
    > > On 07/29, Oleg Nesterov wrote:
    > >>
    > >> On 07/29, Dmitry Adamushko wrote:
    > >> >
    > >> > And I'd say this behavior (of having a partially-created object
    > >> > visible to the outside world) is not that robust. e.g. the
    > >> > aforementioned race would be eliminated if we place a wq on the global
    > >> > list only when it's been successfully initialized.
    > >>
    > >> Yes, we can change __create_workqueue_key() to check err == 0 before
    > >> list_add(),
    > >
    > > Well no, we can't do even this.
    > >
    > > Then we have another race with cpu-hotplug. Suppose we have CPUs 0, 1, 2.
    > > create_workqueue() fails to create cwq->thread for CPU 2 and calls
    > > destroy_workqueue(). Before it takes the cpu_add_remove_lock, _cpu_down()
    > > removes CPU 1 from cpu_populated_map, but since we didn't add this wq
    > > on the global list, cwq[1]->thread remains alive.
    > >
    > > destroy_workqueue() takes cpu_add_remove_lock, and calls
    > > cleanup_workqueue_thread() for CPUs 0 and 2. cwq[1]->thread is lost.
    >
    > Yes, I've actually seen this case and that's why I said "the cleanup
    > path in __create_workqueue_key() would need
    > to be altered" :-) likely, to the extent that it would not be a call
    > to destroy_workqueue() anymore.
    >
    > either something that only does
    >
    > for_each_cpu_mask_nr(cpu, *cpu_map)
    > cleanup_workqueue_thread(per_cpu_ptr(wq->cpu_wq, cpu));
    >
    >
    > and from the _same_ 'cpu_add_remove_lock' section which is used to
    > create a wq (so we don't drop a lock);

    Why should we duplicate the code?

    > _or_ do it outside of the locked section _but_ don't rely on
    > for_each_cpu_mask_nr(cpu, *cpu_map)... e.g. just delete all per-cpu
    > wq->cpu_wq structures that have been initialized (that's no matter if
    > their respective cpus are online/offline now).

    Yes. And this means we change the code to handle another special case:
    destroy() is called by create(). Why?

    > yes, maybe this cleanup path would not look all that fancy (but I
    > didn't try) but I do think that by not exposing "partially-initialized
    > object to the outside world" (e.g. cpu-hotplug events won't see them)
    > this code would become more straightforward and less prone to possible
    > errors/races.
    >
    > e.g. all these "create_workqueue_key() may race with cpu-hotplug" would be gone.

    Once again, from my pov wq is fully initialized. Yes, cwq->thread can
    be NULL or not, and this doesn't necessary match cpu_online_map. This
    is normal, for example CPU_POST_DEAD runs when CPU doesn't exists, but
    cwq[CPU]->thread is alive.

    With the current code we just have no special cases. I do not see
    why create_workqueue_key()->destroy_workqueue() should be special.


    However, I don't claim you are wrong. I think this all is a matter
    of taste. And yes I agree, without the comments the current code
    is not immediately obvious, this probably indicates that my taste
    is not good and you are right ;)

    Oleg.



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