[XEN][PATCH v8 14/19] common/device_tree: Add rwlock for dt_host

[Vikram Garhwal <vikram.garhwal@xxxxxxx>]

 Dynamic programming ops will modify the dt_host and there might be other
 function which are browsing the dt_host at the same time. To avoid the race
 conditions, adding rwlock for browsing the dt_host during runtime. dt_host
 writer will be added in the follow-up patch titled "xen/arm: Implement device
 tree node addition functionalities."

 Reason behind adding rwlock instead of spinlock:
    For now, dynamic programming is the sole modifier of dt_host in Xen during
        run time. All other access functions like iommu_release_dt_device() are
        just reading the dt_host during run-time. So, there is a need to protect
        others from browsing the dt_host while dynamic programming is modifying
        it. rwlock is better suitable for this task as spinlock won't be able to
        differentiate between read and write access.

Signed-off-by: Vikram Garhwal <vikram.garhwal@xxxxxxx>

---
Changes from v7:
    Keep one lock for dt_host instead of lock for each node under dt_host.
---
---
 xen/common/device_tree.c              |  5 +++++
 xen/drivers/passthrough/device_tree.c | 15 +++++++++++++++
 xen/include/xen/device_tree.h         |  6 ++++++
 3 files changed, 26 insertions(+)

diff --git a/xen/common/device_tree.c b/xen/common/device_tree.c
index 0f10037745..6b934fe036 100644
--- a/xen/common/device_tree.c
+++ b/xen/common/device_tree.c
@@ -31,6 +31,7 @@ dt_irq_xlate_func dt_irq_xlate;
 struct dt_device_node *dt_host;
 /* Interrupt controller node*/
 const struct dt_device_node *dt_interrupt_controller;
+rwlock_t dt_host_lock;
 
 /**
  * struct dt_alias_prop - Alias property in 'aliases' node
@@ -2137,7 +2138,11 @@ int unflatten_device_tree(const void *fdt, struct 
dt_device_node **mynodes)
 
     dt_dprintk(" <- unflatten_device_tree()\n");
 
+    /* Init r/w lock for host device tree. */
+    rwlock_init(&dt_host_lock);
+
     return 0;
+
 }
 
 static void dt_alias_add(struct dt_alias_prop *ap,
diff --git a/xen/drivers/passthrough/device_tree.c 
b/xen/drivers/passthrough/device_tree.c
index 4cb32dc0b3..31815d2b60 100644
--- a/xen/drivers/passthrough/device_tree.c
+++ b/xen/drivers/passthrough/device_tree.c
@@ -114,6 +114,8 @@ int iommu_release_dt_devices(struct domain *d)
     if ( !is_iommu_enabled(d) )
         return 0;
 
+    read_lock(&dt_host_lock);
+
     list_for_each_entry_safe(dev, _dev, &hd->dt_devices, domain_list)
     {
         rc = iommu_deassign_dt_device(d, dev);
@@ -121,10 +123,14 @@ int iommu_release_dt_devices(struct domain *d)
         {
             dprintk(XENLOG_ERR, "Failed to deassign %s in domain %u\n",
                     dt_node_full_name(dev), d->domain_id);
+
+            read_unlock(&dt_host_lock);
             return rc;
         }
     }
 
+    read_unlock(&dt_host_lock);
+
     return 0;
 }
 
@@ -251,6 +257,8 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct 
domain *d,
     int ret;
     struct dt_device_node *dev;
 
+    read_lock(&dt_host_lock);
+
     switch ( domctl->cmd )
     {
     case XEN_DOMCTL_assign_device:
@@ -304,7 +312,10 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct 
domain *d,
         spin_unlock(&dtdevs_lock);
 
         if ( d == dom_io )
+        {
+            read_unlock(&dt_host_lock);
             return -EINVAL;
+        }
 
         ret = iommu_add_dt_device(dev);
         if ( ret < 0 )
@@ -342,7 +353,10 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct 
domain *d,
             break;
 
         if ( d == dom_io )
+        {
+            read_unlock(&dt_host_lock);
             return -EINVAL;
+        }
 
         ret = iommu_deassign_dt_device(d, dev);
 
@@ -357,5 +371,6 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct 
domain *d,
         break;
     }
 
+    read_unlock(&dt_host_lock);
     return ret;
 }
diff --git a/xen/include/xen/device_tree.h b/xen/include/xen/device_tree.h
index e507658b23..8191f30197 100644
--- a/xen/include/xen/device_tree.h
+++ b/xen/include/xen/device_tree.h
@@ -18,6 +18,7 @@
 #include <xen/string.h>
 #include <xen/types.h>
 #include <xen/list.h>
+#include <xen/rwlock.h>
 
 #define DEVICE_TREE_MAX_DEPTH 16
 
@@ -216,6 +217,11 @@ extern struct dt_device_node *dt_host;
  */
 extern const struct dt_device_node *dt_interrupt_controller;
 
+/*
+ * Lock that protects r/w updates to unflattened device tree i.e. dt_host.
+ */
+extern rwlock_t dt_host_lock;
+
 /**
  * Find the interrupt controller
  * For the moment we handle only one interrupt controller: the first
-- 
2.17.1