Date:   Tue, 10 Dec 2019 10:39:08 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Vinod Koul <vkoul@...nel.org>, Dave Jiang <dave.jiang@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>
Subject: Re: [PATCH 1/5] dmaengine: Store module owner in dma_device struct



On 2019-12-10 2:53 a.m., Vinod Koul wrote:
> On 22-11-19, 14:42, Dave Jiang wrote:
>>
>>
>> On 11/22/19 2:01 PM, Dan Williams wrote:
>>> On Fri, Nov 22, 2019 at 12:56 PM Logan Gunthorpe <logang@...tatee.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@...el.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.
> 
> Right finally wrapping my head of static dmaengine devices! we can
> indeed have devices going away, but me wondering why this should be
> handled in subsystems! Should the driver core not be doing this instead?
> Would it be not a safe behaviour for unplug to unload the driver and
> thus give a chance to unbind from subsystems too...

Yes, I think it should be in the core. I was just worried about breaking
older drivers. But I'll see if I can move a bit more of the logic for a
v3 series.

Thanks,

Logan

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