linux-kernel - [PATCH v3 1/2] PCI/IOV: Revert "PCI/IOV: Serialize sysfs sriov

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Message-ID: <170795158249.2393.6069168125066679172.stgit@bgt-140510-bm01.eng.stellus.in>
Date: Wed, 14 Feb 2024 22:59:42 +0000
From: Jim Harris <jim.harris@...sung.com>
To: Davidlohr Bueso <dave@...olabs.net>, Jonathan Cameron
	<jonathan.cameron@...wei.com>, Dave Jiang <dave.jiang@...el.com>, "Alison
 Schofield" <alison.schofield@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>, Dan Williams
	<dan.j.williams@...el.com>, Kees Cook <keescook@...omium.org>, "Gustavo A.
 R. Silva" <gustavoars@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-cxl@...r.kernel.org"
	<linux-cxl@...r.kernel.org>, "linux-hardening@...r.kernel.org"
	<linux-hardening@...r.kernel.org>, Leon Romanovsky <leonro@...dia.com>,
	"Jason Gunthorpe" <jgg@...dia.com>, Alex Williamson
	<alex.williamson@...hat.com>, "pierre.cregut@...nge.com"
	<pierre.cregut@...nge.com>
Subject: [PATCH v3 1/2] PCI/IOV: Revert
 "PCI/IOV: Serialize sysfs sriov_numvfs reads vs writes"

If an SR-IOV enabled device is held by vfio, and the device is removed,
vfio will hold device lock and notify userspace of the removal. If
userspace reads the sriov_numvfs sysfs entry, that thread will be blocked
since sriov_numvfs_show() also tries to acquire the device lock. If that
same thread is responsible for releasing the device to vfio, it results in
a deadlock.

The proper way to detect a change to the num_VFs value is to listen for a
sysfs event, not to add a device_lock() on the attribute _show() in the
kernel.

This reverts commit 35ff867b76576e32f34c698ccd11343f7d616204.
Revert had a small conflict, the sprintf() is now changed to sysfs_emit().

Link: https://lore.kernel.org/linux-pci/ZXJI5+f8bUelVXqu@ubuntu/
Suggested-by: Leon Romanovsky <leonro@...dia.com>
Reviewed-by: Leon Romanovsky <leonro@...dia.com>
Signed-off-by: Jim Harris <jim.harris@...sung.com>
---
 drivers/pci/iov.c |    8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
index aaa33e8dc4c9..0ca20cd518d5 100644
--- a/drivers/pci/iov.c
+++ b/drivers/pci/iov.c
@@ -395,14 +395,8 @@ 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 sysfs_emit(buf, "%u\n", num_vfs);
+	return sysfs_emit(buf, "%u\n", pdev->sriov->num_VFs);
 }
 
 /*