Messages in this thread | | | Date | Thu, 12 May 2022 06:56:45 -1000 | From | Tejun Heo <> | Subject | Re: [PATCH v3 (repost)] workqueue: Warn flushing of kernel-global workqueues |
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Hello, Dmitry.
On Thu, May 12, 2022 at 06:13:35AM -0700, Dmitry Torokhov wrote: > > > This means that now the code has to keep track of all work items that it > > > allocated, instead of being able "fire and forget" works (when dealing > > > with extremely infrequent events) and rely on flush_workqueue() to > > > cleanup. > > > > Yes. Moreover, a patch to catch and refuse at compile time was proposed at > > https://lkml.kernel.org/r/738afe71-2983-05d5-f0fc-d94efbdf7634@I-love.SAKURA.ne.jp . > > My comment was not a wholesale endorsement of Tejun's statement, but > rather a note of the fact that it again adds complexity (at least as far > as driver writers are concerned) to the kernel code.
I was more thinking about cases where there are a small number of static work items. If there are multiple dynamic work items, creating a workqueue as a flush domain is the way to go. It does add a bit of complexity but shouldn't be too bad - e.g. it just adds the alloc_workqueue() during init and the exit path can simply use destroy_workqueue() instead of flush.
> > > That flush typically happens in module unload path, and I > > > wonder if the restriction on flush_workqueue() could be relaxed to allow > > > calling it on unload. > > > > A patch for drivers/input/mouse/psmouse-smbus.c is waiting for your response at > > https://lkml.kernel.org/r/25e2b787-cb2c-fb0d-d62c-6577ad1cd9df@I-love.SAKURA.ne.jp . > > Like many modules, flush_workqueue() happens on only module unload in your case. > > Yes, I saw that patch, and that is what prompted my response. I find it > adding complexity and I was wondering if it could be avoided. It also > unclear to me if there is an additional cost coming from allocating a > dedicated workqueue.
A workqueue without WQ_RECLAIM is really cheap. All it does is tracking what's in flight for that particular frontend while interfacing with the shared worker pool.
> I understand that for some of them the change makes sense, but it would > be nice to continue using simple API under limited circumstances.
Hmmm... unfortunately, I can't think of a way to guarantee that a module unloading path can't get involved in a deadlock scenario through system_wq. Given that the added complexity should be something like half a dozen lines of code, switching to separte workqueues feels like the right direction to me.
Thanks.
-- tejun
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