linux-kernel - [PATCH] vfio-pci: Use pci "try" reset interface

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Date:	Mon, 16 Dec 2013 15:16:54 -0700
From:	Alex Williamson <alex.williamson@...hat.com>
To:	alex.williamson@...hat.com
Cc:	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: [PATCH] vfio-pci: Use pci "try" reset interface

PCI resets will attempt to take the device_lock for any device to be
reset.  This is a problem if that lock is already held, for instance
in the device remove path.  It's not sufficient to simply kill the
user process or skip the reset if called after .remove as a race could
result in the same deadlock.  Instead, we handle all resets as "best
effort" using the PCI "try" reset interfaces.  This prevents the user
from being able to induce a deadlock by triggering a reset.

Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
---

This depends on the proposed pci "try" function/slot/bus reset patch.

 drivers/vfio/pci/vfio_pci.c |   29 +++++++++--------------------
 1 file changed, 9 insertions(+), 20 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index 6ab71b9..576e34e 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -139,25 +139,14 @@ static void vfio_pci_disable(struct vfio_pci_device *vdev)
 	pci_write_config_word(pdev, PCI_COMMAND, PCI_COMMAND_INTX_DISABLE);
 
 	/*
-	 * Careful, device_lock may already be held.  This is the case if
-	 * a driver unbind is blocked.  Try to get the locks ourselves to
-	 * prevent a deadlock.
+	 * Try to reset the device.  The success of this is dependent on
+	 * being able to lock the device, which is not always possible.
 	 */
 	if (vdev->reset_works) {
-		bool reset_done = false;
-
-		if (pci_cfg_access_trylock(pdev)) {
-			if (device_trylock(&pdev->dev)) {
-				__pci_reset_function_locked(pdev);
-				reset_done = true;
-				device_unlock(&pdev->dev);
-			}
-			pci_cfg_access_unlock(pdev);
-		}
-
-		if (!reset_done)
-			pr_warn("%s: Unable to acquire locks for reset of %s\n",
-				__func__, dev_name(&pdev->dev));
+		int ret = pci_try_reset_function(pdev);
+		if (ret)
+			pr_debug("%s: reset of device %s = %d\n",
+				__func__, dev_name(&pdev->dev), ret);
 	}
 
 	pci_restore_state(pdev);
@@ -514,7 +503,7 @@ static long vfio_pci_ioctl(void *device_data,
 
 	} else if (cmd == VFIO_DEVICE_RESET) {
 		return vdev->reset_works ?
-			pci_reset_function(vdev->pdev) : -EINVAL;
+			pci_try_reset_function(vdev->pdev) : -EINVAL;
 
 	} else if (cmd == VFIO_DEVICE_GET_PCI_HOT_RESET_INFO) {
 		struct vfio_pci_hot_reset_info hdr;
@@ -684,8 +673,8 @@ reset_info_exit:
 						    &info, slot);
 		if (!ret)
 			/* User has access, do the reset */
-			ret = slot ? pci_reset_slot(vdev->pdev->slot) :
-				     pci_reset_bus(vdev->pdev->bus);
+			ret = slot ? pci_try_reset_slot(vdev->pdev->slot) :
+				     pci_try_reset_bus(vdev->pdev->bus);
 
 hot_reset_release:
 		for (i--; i >= 0; i--)

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