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

[John Hubbard <jhubbard@nvidia.com>]

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