Re: [PATCH 1/5] dmaengine: Store module owner in dma_device struct

[Dave Jiang]

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