Messages in this thread | | | Date | Wed, 23 Feb 2022 17:05:23 +0000 | Subject | Re: [PATCH v6 02/11] driver core: Add dma_cleanup callback in bus_type | From | Robin Murphy <> |
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On 2022-02-23 16:03, Greg Kroah-Hartman wrote: > On Wed, Feb 23, 2022 at 10:30:11AM -0400, Jason Gunthorpe wrote: >> 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. > > "other modules" should never care if a device has a driver bound to it > because instantly after the check happens, it can change so what ever > logic it wanted to do with that knowledge is gone. > > Unless the bus lock is held that the device is on, but that should be > only accessable from within the driver core as it controls that type of > stuff, not any random other part of the kernel. > > And in looking at this, ick, there are loads of places in the kernel > that are thinking that this pointer being set to something actually > means something. Sometimes it does, but lots of places, it doesn't as > it can change.
That's fine. In this case we're only talking about the low-level IOMMU code which has to be in cahoots with the driver core to some degree (via these new callbacks) anyway, but if you're uncomfortable about relying on dev->driver even there, I can live with that. There are several potential places to capture the relevant information in IOMMU API private data, from the point in really_probe() where it *is* stable, and then never look at dev->driver ever again - even from .dma_cleanup() or future equivalent, which is the aspect from whence this whole proof-of-concept tangent span out.
Cheers, Robin.
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