linux-kernel - Re: [PATCH 1/5] dmaengine: Store module owner in dma

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Date:   Fri, 22 Nov 2019 09:53:23 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     Vinod Koul <vkoul@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 1/5] dmaengine: Store module owner in dma_device struct



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.

>>
>> Essentially, at any time, a user can do this:
>>
>> echo 0000:83:00.4 > /sys/bus/pci/drivers/plx_dma/unbind
>>
>> Which will call plx_dma_remove() regardless of whether anyone has a
>> reference to the module, and regardless of whether the dma channel is
>> currently in use. I feel it is important that drivers support this
>> without crashing, and my plx_dma driver does the correct thing here.
>>
>> Logan
>