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

[Bjorn Helgaas]

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@xxxxxxxxxxxx>
> ---
>  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.

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 *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?