[RFC PATCH v2 02/11] PCI/P2PDMA: Avoid pci_get_slot() which sleeps

[Logan Gunthorpe]

On 2021-03-12 1:57 p.m., Bjorn Helgaas wrote:
> On Thu, Mar 11, 2021 at 04:31:32PM -0700, Logan Gunthorpe wrote:
>> In order to use upstream_bridge_distance_warn() from a dma_map function,
>> it must not sleep. However, pci_get_slot() takes the pci_bus_sem so it
>> might sleep.
>>
>> In order to avoid this, try to get the host bridge's device from
>> bus->self, and if that is not set just get the first element in the
>> list. It should be impossible for the host bridges device to go away
>> while references are held on child devices, so the first element
>> should not change and this should be safe.
>>
>> Signed-off-by: Logan Gunthorpe <logang at deltatee.com>
>> ---
>>  drivers/pci/p2pdma.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
>> index bd89437faf06..2135fe69bb07 100644
>> --- a/drivers/pci/p2pdma.c
>> +++ b/drivers/pci/p2pdma.c
>> @@ -311,11 +311,15 @@ static const struct pci_p2pdma_whitelist_entry {
>>  static bool __host_bridge_whitelist(struct pci_host_bridge *host,
>>  				    bool same_host_bridge)
>>  {
>> -	struct pci_dev *root = pci_get_slot(host->bus, PCI_DEVFN(0, 0));
>>  	const struct pci_p2pdma_whitelist_entry *entry;
>> +	struct pci_dev *root = host->bus->self;
>>  	unsigned short vendor, device;
>>  
>>  	if (!root)
>> +		root = list_first_entry_or_null(&host->bus->devices,
>> +						struct pci_dev, bus_list);
> 
> Replacing one ugliness (assuming there is a pci_dev for the host
> bridge, and that it is at 00.0) with another (still assuming a pci_dev
> and that it is host->bus->self or the first entry).  I can't suggest
> anything better, but maybe a little comment in the code would help
> future readers.

Yeah, I struggled to find a solution here; this was the best I could
come up with. I'd love it if someone had a better idea. I can add a
comment for future iterations.

> I wish we had a real way to discover this property without the
> whitelist, at least for future devices.  Was there ever any interest
> in a _DSM or similar interface for this?

I'd also like to get rid of the whitelist, but I have no idea how or who
would have to lead a fight to get the hardware to self describe in way
that we could use.

> I *am* very glad to remove a pci_get_slot() usage.
> 
>> +
>> +	if (!root || root->devfn)
>>  		return false;
>>  
>>  	vendor = root->vendor;
> 
> Don't you need to also remove the "pci_dev_put(root)" a few lines
> below?

Yes, right. Good catch!

Logan