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

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