From: Johannes Berg <firstname.lastname@example.org> To: Bart Van Assche <email@example.com>, Tejun Heo <firstname.lastname@example.org> Cc: "email@example.com" <firstname.lastname@example.org>, Christoph Hellwig <email@example.com>, Sagi Grimberg <firstname.lastname@example.org>, "email@example.com" <firstname.lastname@example.org> Subject: Re: [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint Date: Thu, 25 Oct 2018 19:02:29 +0200 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <firstname.lastname@example.org> On Thu, 2018-10-25 at 15:05 +0000, Bart Van Assche wrote: > It can happen that the direct I/O queue creates and destroys an empty > workqueue from inside a work function. So, thinking about this more, can you guarantee (somehow) that the workqueue is empty at this point? Perhaps then we should encode that guarantee into the API, and actually make it WARN_ON() if something is scheduled on it at that point? And then skip lockdep in this case. You've actually done a superset of this here - you've basically said this workqueue was never used, not just that it's empty right now. However, if you could guarantee that it was never used at this point, I guess I fail to see what you need it for anyway (or alternatively, why you wouldn't allocate it later after this point.) It does seem strange to me to make lockdep contingent on "has this workqueue been used" because there may be things queued on it that only happen in certain cases (hardware, phases of the moon ;-) ) and lockdep sort of says "once you execute all the paths that _could_ lead to a deadlock, I'll warn you about it, even if it never happened". It seems that we'd want to extend that to workqueues. Also, I think in a way what this does is try to differentiate the classes of workqueues: used vs. not used per instance, but I'm not really sure that's what you want - after all, could it really never have been used? Surely it has a purpose? But anyway, I'll need to take a better look at the lockdep splat later (i.e. when the kids are sleeping.) Do you know how to reproduce this? johannes
next prev parent reply other threads:[~2018-10-25 17:02 UTC|newest] Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-10-25 15:05 [PATCH 0/3] Suppress false positives triggered by workqueue lockdep annotations Bart Van Assche 2018-10-25 15:05 ` [PATCH 1/3] kernel/workqueue: Remove lockdep annotation from __flush_work() Bart Van Assche 2018-10-25 15:31 ` Johannes Berg 2018-10-25 15:57 ` Johannes Berg 2018-10-25 16:01 ` Bart Van Assche 2018-10-25 15:05 ` [PATCH 2/3] kernel/workqueue: Surround work execution with shared lock annotations Bart Van Assche 2018-10-25 16:53 ` Johannes Berg 2018-10-25 17:22 ` Bart Van Assche 2018-10-25 19:17 ` Johannes Berg 2018-10-25 15:05 ` [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint Bart Van Assche 2018-10-25 15:34 ` Johannes Berg 2018-10-25 15:55 ` Bart Van Assche 2018-10-25 19:59 ` Johannes Berg 2018-10-25 20:21 ` Theodore Y. Ts'o 2018-10-25 20:26 ` Johannes Berg 2018-10-25 15:36 ` Tejun Heo 2018-10-25 15:37 ` Tejun Heo 2018-10-25 20:13 ` Johannes Berg 2018-10-25 15:40 ` Theodore Y. Ts'o 2018-10-25 17:02 ` Johannes Berg [this message] 2018-10-25 17:11 ` Bart Van Assche 2018-10-25 19:51 ` Johannes Berg 2018-10-25 20:39 ` Bart Van Assche 2018-10-25 20:47 ` Johannes Berg 2018-10-25 15:27 ` [PATCH 0/3] Suppress false positives triggered by workqueue lockdep annotations Johannes Berg 2018-10-25 15:47 ` Bart Van Assche
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