Messages in this thread | | | Subject | Re: [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint | From | Johannes Berg <> | Date | Thu, 25 Oct 2018 22:26:04 +0200 |
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On Thu, 2018-10-25 at 16:21 -0400, Theodore Y. Ts'o wrote:
> We can guarantee it from someone who is looking at the code path. In > dio_set_defer_completion:
[snip]
Right, it's indeed pretty obvious. I shouldn't have tried to reply before the kids went to bed, that made me cut some corners ;-)
> The race found in the syzbot reproducer has multiple threads all > running DIO writes at the same time. So we have multiple threads > calling sb_init_dio_done_wq, but all but one will lose the race, and > then call destry_workqueue on the freshly created (but never used) > workqueue.
Right.
> We could replace the destroy_workqueue(wq) with a > "I_solemnly_swear_this_workqueue_has_never_been_used_please_destroy(wq)".
:-)
> Or, as Tejun suggested, "destroy_workqueue_skip_drain(wq)", but there is > no way for the workqueue code to know whether the caller was using the > interface correctly. So this basically becomes a philosophical > question about whether or not we trust the caller to be correct or > not.
Right. Same with the lockdep annotation I suggested over in my other email, of course. I think that the set of APIs I wrote there ({drain,flush,destroy}_workqueue_nested()) might be more generally useful in other cases, not just this one, and I suspect that this code would basically be the only user of destroy_workqueue_skip_drain().
> I don't see an obvious way that we can test to make sure the workqueue > is never used without actually taking a performance. Am I correct > that we would need to take the wq->mutex before we can mess with the > wq->flags field?
I don't really know, sorry.
johannes
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