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To:     Vinod Koul <vkoul@kernel.org>
Cc:     linux-kernel@vger.kernel.org, dmaengine@vger.kernel.org,
        Dan Williams <dan.j.williams@intel.com>
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From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <fa45de06-089f-367c-7816-2ee040e41d24@deltatee.com>
Date:   Thu, 14 Nov 2019 10:03:05 -0700
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Subject: Re: [PATCH 1/5] dmaengine: Store module owner in dma_device struct
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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.

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