Messages in this thread |  | | | Date | Tue, 29 Jul 2008 16:20:28 +0200 | | From | "Dmitry Adamushko" <> | | Subject | Re: [patch, minor] workqueue: consistently use 'err' in __create_workqueue_key() |
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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);
_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, 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.
-- Best regards, Dmitry Adamushko
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