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Re: [libvirt] [PATCH 1/1] Change locking for udev monitor and callbacks



On Tue, Apr 05, 2011 at 04:14:17PM -0500, Serge Hallyn wrote:
> We're seeing bugs apparently resulting from thread unsafety of
> libpciaccess, such as
> https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/726099
> To prevent those, as suggested by danpb on irc, move the
> nodeDeviceLock(driverState) higher into the callers.  In
> particular:
> 
>   udevDeviceMonitorStartup should hold the lock while calling
>   udevEnumerateDevices(), and udevEventHandleCallback should hold it
>   over its entire execution.
> 
> It's not clear to me whether it is ok to hold the
> nodeDeviceLock while taking the virNodeDeviceObjLock(dev) on a
> device.  If not, then the lock will need to be dropped around
> the calling of udevSetupSystemDev(), and udevAddOneDevice()
> may not actually be safe to call from higher layers with the
> driverstate lock held.
> 
> libvirt 0.8.8 with this patch on it seems to work fine for me.
> Assuming it looks ok and I haven't done anything obviously dumb,
> I'll ask the bug submitters to try this patch.
> 
> Signed-off-by: Serge Hallyn <serge hallyn ubuntu com>
> ---
>  src/node_device/node_device_udev.c |   21 +++++++++------------
>  1 files changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/src/node_device/node_device_udev.c b/src/node_device/node_device_udev.c
> index 372f1d1..2139ef3 100644
> --- a/src/node_device/node_device_udev.c
> +++ b/src/node_device/node_device_udev.c
> @@ -1202,7 +1202,6 @@ static int udevRemoveOneDevice(struct udev_device *device)
>      int ret = 0;
>  
>      name = udev_device_get_syspath(device);
> -    nodeDeviceLock(driverState);
>      dev = virNodeDeviceFindBySysfsPath(&driverState->devs, name);
>  
>      if (dev != NULL) {
> @@ -1214,7 +1213,6 @@ static int udevRemoveOneDevice(struct udev_device *device)
>                    name);
>          ret = -1;
>      }
> -    nodeDeviceUnlock(driverState);
>  
>      return ret;
>  }
> @@ -1316,9 +1314,7 @@ static int udevAddOneDevice(struct udev_device *device)
>  
>      /* If this is a device change, the old definition will be freed
>       * and the current definition will take its place. */
> -    nodeDeviceLock(driverState);
>      dev = virNodeDeviceAssignDef(&driverState->devs, def);
> -    nodeDeviceUnlock(driverState);
>  
>      if (dev == NULL) {
>          VIR_ERROR(_("Failed to create device for '%s'"), def->name);
> @@ -1442,6 +1438,7 @@ static void udevEventHandleCallback(int watch ATTRIBUTE_UNUSED,
>      const char *action = NULL;
>      int udev_fd = -1;
>  
> +    nodeDeviceLock(driverState);
>      udev_fd = udev_monitor_get_fd(udev_monitor);
>      if (fd != udev_fd) {
>          VIR_ERROR(_("File descriptor returned by udev %d does not "
> @@ -1470,6 +1467,7 @@ static void udevEventHandleCallback(int watch ATTRIBUTE_UNUSED,
>  
>  out:
>      udev_device_unref(device);
> +    nodeDeviceUnlock(driverState);
>      return;
>  }
>  
> @@ -1647,10 +1645,9 @@ static int udevDeviceMonitorStartup(int privileged)
>      priv->udev_monitor = udev_monitor_new_from_netlink(udev, "udev");
>      if (priv->udev_monitor == NULL) {
>          VIR_FREE(priv);
> -        nodeDeviceUnlock(driverState);
>          VIR_ERROR0(_("udev_monitor_new_from_netlink returned NULL"));
>          ret = -1;
> -        goto out;
> +        goto out_unlock;
>      }
>  
>      udev_monitor_enable_receiving(priv->udev_monitor);
> @@ -1670,26 +1667,26 @@ static int udevDeviceMonitorStartup(int privileged)
>                                      VIR_EVENT_HANDLE_READABLE,
>                                      udevEventHandleCallback, NULL, NULL);
>      if (priv->watch == -1) {
> -        nodeDeviceUnlock(driverState);
>          ret = -1;
> -        goto out;
> +        goto out_unlock;
>      }
>  
> -    nodeDeviceUnlock(driverState);
> -
>      /* Create a fictional 'computer' device to root the device tree. */
>      if (udevSetupSystemDev() != 0) {
>          ret = -1;
> -        goto out;
> +        goto out_unlock;
>      }
>  
>      /* Populate with known devices */
>  
>      if (udevEnumerateDevices(udev) != 0) {
>          ret = -1;
> -        goto out;
> +        goto out_unlock;
>      }
>  
> +out_unlock:
> +    nodeDeviceUnlock(driverState);
> +
>  out:
>      if (ret == -1) {
>          udevDeviceMonitorShutdown();

ACK, it looks good to me - serializing all access to the udev
and libpciaccess APIs is a good conservative approach.

Daniel
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