Messages in this thread | | | Subject | Re: [PATCH 1/5] dmaengine: Store module owner in dma_device struct | From | Dave Jiang <> | Date | Fri, 22 Nov 2019 14:42:39 -0700 |
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On 11/22/19 2:01 PM, Dan Williams wrote: > On Fri, Nov 22, 2019 at 12:56 PM Logan Gunthorpe <logang@deltatee.com> wrote: >> >> >> >> On 2019-11-22 1:50 p.m., Dan Williams wrote: >>> On Fri, Nov 22, 2019 at 8:53 AM Dave Jiang <dave.jiang@intel.com> wrote: >>>> >>>> >>>> >>>> On 11/21/19 10:20 PM, Vinod Koul wrote: >>>>> On 14-11-19, 10:03, Logan Gunthorpe wrote: >>>>>> >>>>>> >>>>>> On 2019-11-13 9:55 p.m., Vinod Koul wrote: >>>>>>>> But that's the problem. We can't expect our users to be "nice" and not >>>>>>>> unbind when the driver is in use. Killing the kernel if the user >>>>>>>> unexpectedly unbinds is not acceptable. >>>>>>> >>>>>>> And that is why we review the code and ensure this does not happen and >>>>>>> behaviour is as expected >>>>>> >>>>>> Yes, but the current code can kill the kernel when the driver is unbound. >>>>>> >>>>>>>>>> I suspect this is less of an issue for most devices as they wouldn't >>>>>>>>>> normally be unbound while in use (for example there's really no reason >>>>>>>>>> to ever unbind IOAT seeing it's built into the system). Though, the fact >>>>>>>>>> is, the user could unbind these devices at anytime and we don't want to >>>>>>>>>> panic if they do. >>>>>>>>> >>>>>>>>> There are many drivers which do modules so yes I am expecting unbind and >>>>>>>>> even a bind following that to work >>>>>>>> >>>>>>>> Except they will panic if they unbind while in use, so that's a >>>>>>>> questionable definition of "work". >>>>>>> >>>>>>> dmaengine core has module reference so while they are being used they >>>>>>> won't be removed (unless I complete misread the driver core behaviour) >>>>>> >>>>>> Yes, as I mentioned in my other email, holding a module reference does >>>>>> not prevent the driver from being unbound. Any driver can be unbound by >>>>>> the user at any time without the module being removed. >>>>> >>>>> That sounds okay then. >>>> >>>> I'm actually glad Logan is putting some work in addressing this. I also >>>> ran into the same issue as well dealing with unbinds on my new driver. >>> >>> This was an original mistake of the dmaengine implementation that >>> Vinod inherited. Module pinning is distinct from preventing device >>> unbind which ultimately can't be prevented. Longer term I think we >>> need to audit dmaengine consumers to make sure they are prepared for >>> the driver to be removed similar to how other request based drivers >>> can gracefully return an error status when the device goes away rather >>> than crashing. >> >> Yes, but that will be a big project because there are a lot of drivers. > > Oh yes, in fact I think it's something that can only reasonably be > considered for new consumers. > >> But I think the dmaengine common code needs to support removal properly, >> which essentially means changing how all the drivers allocate and free >> their structures, among other things. >> >> The one saving grace is that most of the drivers are for SOCs which >> can't be physically removed and there's really no use-case for the user >> to call unbind. > > Yes, the SOC case is not so much my concern as the generic offload use > cases, especially if those offloads are in a similar hotplug domain as > a cpu. >
It becomes a bigger issue when "channels" can be reconfigured and can come and go in a hot plug fashion.
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