PCI/IOV: Serialize sysfs sriov_numvfs reads vs writes

When sriov_numvfs is being updated, we call the driver->sriov_configure()
function, which may enable VFs and call probe functions, which may make new
devices visible.  This all happens before before sriov_numvfs_store()
updates sriov->num_VFs, so previously, concurrent sysfs reads of
sriov_numvfs returned stale values.

Serialize the sysfs read vs the write so the read returns the correct
num_VFs value.

[bhelgaas: hold device_lock instead of checking mutex_is_locked()]
Link: https://bugzilla.kernel.org/show_bug.cgi?id=202991
Link: https://lore.kernel.org/r/20190911072736.32091-1-pierre.cregut@orange.comSigned-off-by: Pierre Crégut <pierre.cregut@orange.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/iov.c

@@ -254,8 +254,14 @@ static ssize_t sriov_numvfs_show(struct device *dev,
 				 char *buf)
 {
 	struct pci_dev *pdev = to_pci_dev(dev);
+	u16 num_vfs;
+
+	/* Serialize vs sriov_numvfs_store() so readers see valid num_VFs */
+	device_lock(&pdev->dev);
+	num_vfs = pdev->sriov->num_VFs;
+	device_unlock(&pdev->dev);
 
-	return sprintf(buf, "%u\n", pdev->sriov->num_VFs);
+	return sprintf(buf, "%u\n", num_vfs);
 }
 
 /*