linux-kernel - [PATCH 02/16] PCI/P2PDMA: Avoid pci_get

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Date:   Thu,  8 Apr 2021 11:01:09 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Logan Gunthorpe <logang@...tatee.com>
Subject: [PATCH 02/16] PCI/P2PDMA: Avoid pci_get_slot() which sleeps

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.

Signed-off-by: Logan Gunthorpe <logang@...tatee.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)
-- 
2.20.1