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

Don Dutile
2021-05-11T16:05:51Z

On 4/8/21 1:01 PM, 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
> device list. It should be impossible for the host bridge's device to
> go away while references are held on child devices, so the first element
> should not be able to change and, thus, this should be safe.
Bjorn:
Why wouldn't (shouldn't?) the bus->self field be set for a host bridge device?
Should this situation be repaired in the host-brige config/setup code elsewhere in the kernel.
... and here, a check-and-fail with info of what doesn't have it setup (another new pci function to do the check & prinfo), so it can point to the offending host-bridge, and thus, the code that needs to be updated?

> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> ---
> drivers/pci/p2pdma.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index bd89437faf06..473a08940fbc 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -311,16 +311,26 @@ 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;
>
> + /*
> + * This makes the assumption that the first device on the bus is the
> + * bridge itself and it has the devfn of 00.0. This assumption should
> + * hold for the devices in the white list above, and if there are cases
> + * where this isn't true they will have to be dealt with when such a
> + * case is added to the whitelist.
> + */
> if (!root)
> + root = list_first_entry_or_null(&host->bus->devices,
> + struct pci_dev, bus_list);
> +
> + if (!root || root->devfn)
> return false;
>
> vendor = root->vendor;
> device = root->device;
> - pci_dev_put(root);
>
> for (entry = pci_p2pdma_whitelist; entry->vendor; entry++) {
> if (vendor != entry->vendor || device != entry->device)