Re: [PATCH 02/16] PCI/P2PDMA: Avoid pci_get_slot() which sleeps

[John Hubbard]

On 5/3/21 9:08 AM, Logan Gunthorpe wrote:
...
> > By the way, pre-existing code comment: pci_p2pdma_whitelist[] seems
> > really short. From a naive point of view, I'd expect that there must be
> > a lot more CPUs/chipsets that can do pci p2p, what do you think? I
> > wonder if we have to be so super strict, anyway. It just seems extremely
> > limited, and I suspect there will be some additions to the list as soon
> > as we start to use this.
>
> Yes, well unfortunately we have no other way to determine what host
> bridges can communicate with P2P. We settled on a whitelist when the
> series was first patch. Nobody likes that situation, but nobody has
> found anything better. We've been hoping standards bodies would give us
> a flag but I haven't heard anything about that. At least AMD has been
> able to guarantee us that all CPUs newer than Zen will support so that
> covers a large swath. It would be nice if we could say something similar
> for Intel.

Thanks for explaining the situation!

> > OK, yes this avoids taking the pci_bus_sem, but it's kind of cheating.
> > Why is it OK to avoid taking any locks in order to retrieve the
> > first entry from the list, but in order to retrieve any other entry, you
> > have to aquire the pci_bus_sem, and get a reference as well? Something
> > is inconsistent there.
> >
> > The new version here also no longer takes a reference on the device,
> > which is also cheating. But I'm guessing that the unstated assumption
> > here is that there is always at least one entry in the list. But if
> > that's true, then it's better to show clearly that assumption, instead
> > of hiding it in an implicit call that skips both locking and reference
> > counting.
>
> Because we hold a reference to a child device of the bus. So the host
> bus device can't go away until the child device has been released. An
> earlier version of the P2PDMA patchset had a lot more extraneous get
> device calls until someone else pointed this out.
>
> > You could add a new function, which is a cut-down version of pci_get_slot(),
> > like this, and call this from __host_bridge_whitelist():
> >
> > /*
> >    * A special purpose variant of pci_get_slot() that doesn't take the pci_bus_sem
> >    * lock, and only looks for the 00.0 bus-device-function. Once the PCI bus is
> >    * up, it is safe to call this, because there will always be a top-level PCI
> >    * root device.
> >    *
> >    * Other assumptions: the root device is the first device in the list, and the
> >    * root device is numbered 00.0.
> >    */
> > struct pci_dev *pci_get_root_slot(struct pci_bus *bus)
> > {
> > 	struct pci_dev *root;
> > 	unsigned devfn = PCI_DEVFN(0, 0);
> >
> > 	root = list_first_entry_or_null(&bus->devices, struct pci_dev,
> > 					bus_list);
> > 	if (root->devfn == devfn)
> > 		goto out;
> >
> > 	root = NULL;
> >    out:
> > 	pci_dev_get(root);
> > 	return root;
> > }
> > EXPORT_SYMBOL(pci_get_root_slot);
> >
> > ...I think that's a lot clearer to the reader, about what's going on here.
>
> Per above, I think the reference count is unnecessary. But I could wrap
> it in a static function for clarity. (There's no reason to export this
> function).

Yes, please.


thanks,
--
John Hubbard
NVIDIA