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Re: [Xen-devel] [PATCH v4 2/5] xen/pciback: Don't deadlock when unbinding.



On 07/14/2014 10:13 AM, Konrad Rzeszutek Wilk wrote:
On Fri, Jul 11, 2014 at 05:02:01PM -0400, Konrad Rzeszutek Wilk wrote:
--- a/drivers/xen/xen-pciback/pci_stub.c
+++ b/drivers/xen/xen-pciback/pci_stub.c
@@ -250,6 +250,8 @@ struct pci_dev *pcistub_get_pci_dev(struct xen_pcibk_device 
*pdev,
   *  - 'echo BDF > unbind' with a guest still using it. See pcistub_remove
   *
   *  As such we have to be careful.
+ *
+ *  To make this easier, the caller has to hold the device lock.
Should we assert that the lock is being held?
Yes of course we should. Thank you!
How about this:

 From 388a03c598218dac8bfeb6c5bf3992e0d1e37d1e Mon Sep 17 00:00:00 2001
From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Date: Tue, 8 Jul 2014 11:12:02 -0400
Subject: [PATCH] xen/pciback: Don't deadlock when unbinding.

As commit 0a9fd0152929db372ff61b0d6c280fdd34ae8bdb
'xen/pciback: Document the entry points for 'pcistub_put_pci_dev''
explained there are four entry points in this function.
Two of them are when the user fiddles in the SysFS to
unbind a device which might be in use by a guest or not.

Both 'unbind' states will cause a deadlock as the the PCI lock has
already been taken, which then pci_device_reset tries to take.

We can simplify this by requiring that all callers of
pcistub_put_pci_dev MUST hold the device lock. And then
we can just call the lockless version of pci_device_reset.

To make it even simpler we will modify xen_pcibk_release_pci_dev
to quality whether it should take a lock or not - as it ends
up calling xen_pcibk_release_pci_dev and needs to hold the lock.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
[v2: Per David Vrabel's suggestion - use lockless version of reset]
[v3: Per Boris suggestion add assertion mechanism]
---
  drivers/xen/xen-pciback/passthrough.c |  9 +++++++--
  drivers/xen/xen-pciback/pci_stub.c    | 12 ++++++------
  drivers/xen/xen-pciback/pciback.h     |  7 ++++---
  drivers/xen/xen-pciback/vpci.c        |  9 +++++++--
  drivers/xen/xen-pciback/xenbus.c      |  2 +-
  5 files changed, 25 insertions(+), 14 deletions(-)

diff --git a/drivers/xen/xen-pciback/passthrough.c 
b/drivers/xen/xen-pciback/passthrough.c
index 828dddc..d0c3fb4 100644
--- a/drivers/xen/xen-pciback/passthrough.c
+++ b/drivers/xen/xen-pciback/passthrough.c
@@ -69,7 +69,7 @@ static int __xen_pcibk_add_pci_dev(struct xen_pcibk_device 
*pdev,
  }
static void __xen_pcibk_release_pci_dev(struct xen_pcibk_device *pdev,
-                                       struct pci_dev *dev)
+                                       struct pci_dev *dev, bool lock)
  {
        struct passthrough_dev_data *dev_data = pdev->pci_dev_data;
        struct pci_dev_entry *dev_entry, *t;
@@ -87,8 +87,13 @@ static void __xen_pcibk_release_pci_dev(struct 
xen_pcibk_device *pdev,
mutex_unlock(&dev_data->lock); - if (found_dev)
+       if (found_dev) {
+               if (lock)
+                       device_lock(&found_dev->dev);
                pcistub_put_pci_dev(found_dev);
+               if (lock)
+                       device_unlock(&found_dev->dev);
+       }
  }
static int __xen_pcibk_init_devices(struct xen_pcibk_device *pdev)
diff --git a/drivers/xen/xen-pciback/pci_stub.c 
b/drivers/xen/xen-pciback/pci_stub.c
index d57a173..8293fbb 100644
--- a/drivers/xen/xen-pciback/pci_stub.c
+++ b/drivers/xen/xen-pciback/pci_stub.c
@@ -250,6 +250,8 @@ struct pci_dev *pcistub_get_pci_dev(struct xen_pcibk_device 
*pdev,
   *  - 'echo BDF > unbind' with a guest still using it. See pcistub_remove
   *
   *  As such we have to be careful.
+ *
+ *  To make this easier, the caller has to hold the device lock.
   */
  void pcistub_put_pci_dev(struct pci_dev *dev)
  {
@@ -276,11 +278,8 @@ void pcistub_put_pci_dev(struct pci_dev *dev)
        /* Cleanup our device
         * (so it's ready for the next domain)
         */
-
-       /* This is OK - we are running from workqueue context
-        * and want to inhibit the user from fiddling with 'reset'
-        */
-       pci_reset_function(dev);
+       lockdep_assert_held(&dev->dev.mutex);
+       __pci_reset_function_locked(dev);
        pci_restore_state(dev);

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

(Although I wonder about the fact that we are exposing the mutex which is typically hidden by device_lock()/unlock() inlines. Have you considered adding something like is_device_locked() to device.h?)

-boris


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