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

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Message-ID: <20191122052010.GO82508@vkoul-mobl>
Date:   Fri, 22 Nov 2019 10:50:10 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     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 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.
> 
> 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

-- 
~Vinod