Re: [PATCH v6 02/11] driver core: Add dma_cleanup callback in bus_type

Jason Gunthorpe
2022-02-23T14:35:21Z

On Wed, Feb 23, 2022 at 10:09:01AM -0400, Jason Gunthorpe wrote:
> On Wed, Feb 23, 2022 at 03:06:35PM +0100, Greg Kroah-Hartman wrote:
> > On Wed, Feb 23, 2022 at 09:46:27AM -0400, Jason Gunthorpe wrote:
> > > On Wed, Feb 23, 2022 at 01:04:00PM +0000, Robin Murphy wrote:
> > >
> > > > 1 - tmp->driver is non-NULL because tmp is already bound.
> > > > 1.a - If tmp->driver->driver_managed_dma == 0, the group must currently be
> > > > DMA-API-owned as a whole. Regardless of what driver dev has unbound from,
> > > > its removal does not release someone else's DMA API (co-)ownership.
> > >
> > > This is an uncommon locking pattern, but it does work. It relies on
> > > the mutex being an effective synchronization barrier for an unlocked
> > > store:
> > >
> > > WRITE_ONCE(dev->driver, NULL)
> >
> > Only the driver core should be messing with the dev->driver pointer as
> > when it does so, it already has the proper locks held. Do I need to
> > move that to a "private" location so that nothing outside of the driver
> > core can mess with it?
>
> It would be nice, I've seen a abuse and mislocking of it in drivers

Though to be clear, what Robin is describing is still keeping the
dev->driver stores in dd.c, just reading it in a lockless way from
other modules.

Jason