[Xen-devel] [PATCH v12 2/5] xenbus/backend: Protect xenbus callback with lock

[SeongJae Park <sjpark@xxxxxxxxxx>]

From: SeongJae Park <sjpark@xxxxxxxxx>

'reclaim_memory' callback can race with a driver code as this callback
will be called from any memory pressure detected context.  To deal with
the case, this commit adds a spinlock in the 'xenbus_device'.  Whenever
'reclaim_memory' callback is called, the lock of the device which passed
to the callback as its argument is locked.  Thus, drivers registering
their 'reclaim_memory' callback should protect the data that might race
with the callback with the lock by themselves.

Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
---
 drivers/xen/xenbus/xenbus_probe.c         |  1 +
 drivers/xen/xenbus/xenbus_probe_backend.c | 11 +++++++++--
 include/xen/xenbus.h                      |  2 ++
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/xenbus/xenbus_probe.c 
b/drivers/xen/xenbus/xenbus_probe.c
index 5b471889d723..b86393f172e6 100644
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -472,6 +472,7 @@ int xenbus_probe_node(struct xen_bus_type *bus,
                goto fail;
 
        dev_set_name(&xendev->dev, "%s", devname);
+       spin_lock_init(&xendev->reclaim_lock);
 
        /* Register with generic device framework. */
        err = device_register(&xendev->dev);
diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c 
b/drivers/xen/xenbus/xenbus_probe_backend.c
index 7e78ebef7c54..e862cb932cc4 100644
--- a/drivers/xen/xenbus/xenbus_probe_backend.c
+++ b/drivers/xen/xenbus/xenbus_probe_backend.c
@@ -251,12 +251,19 @@ static int backend_probe_and_watch(struct notifier_block 
*notifier,
 static int backend_reclaim_memory(struct device *dev, void *data)
 {
        const struct xenbus_driver *drv;
+       struct xenbus_device *xdev;
+       unsigned long flags;
 
        if (!dev->driver)
                return 0;
        drv = to_xenbus_driver(dev->driver);
-       if (drv && drv->reclaim_memory)
-               drv->reclaim_memory(to_xenbus_device(dev));
+       if (drv && drv->reclaim_memory) {
+               xdev = to_xenbus_device(dev);
+               if (!spin_trylock_irqsave(&xdev->reclaim_lock, flags))
+                       return 0;
+               drv->reclaim_memory(xdev);
+               spin_unlock_irqrestore(&xdev->reclaim_lock, flags);
+       }
        return 0;
 }
 
diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index c861cfb6f720..d9468313061d 100644
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -76,6 +76,8 @@ struct xenbus_device {
        enum xenbus_state state;
        struct completion down;
        struct work_struct work;
+       /* 'reclaim_memory' callback is called while this lock is acquired */
+       spinlock_t reclaim_lock;
 };
 
 static inline struct xenbus_device *to_xenbus_device(struct device *dev)
-- 
2.17.1


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