[PATCH v2 1/2] xen/common/dt-overlay: Fix missing lock when remove the device

[Henry Wang <xin.wang2@xxxxxxx>]

If CONFIG_DEBUG=y, below assertion will be triggered:
(XEN) Assertion 'rw_is_locked(&dt_host_lock)' failed at 
drivers/passthrough/device_tree.c:146
(XEN) ----[ Xen-4.19-unstable  arm64  debug=y  Not tainted ]----
(XEN) CPU:    0
(XEN) PC:     00000a0000257418 iommu_remove_dt_device+0x8c/0xd4
(XEN) LR:     00000a00002573a0
(XEN) SP:     00008000fff7fb30
(XEN) CPSR:   0000000000000249 MODE:64-bit EL2h (Hypervisor, handler)
[...]

(XEN) Xen call trace:
(XEN)    [<00000a0000257418>] iommu_remove_dt_device+0x8c/0xd4 (PC)
(XEN)    [<00000a00002573a0>] iommu_remove_dt_device+0x14/0xd4 (LR)
(XEN)    [<00000a000020797c>] dt-overlay.c#remove_node_resources+0x8c/0x90
(XEN)    [<00000a0000207f14>] dt-overlay.c#remove_nodes+0x524/0x648
(XEN)    [<00000a0000208460>] dt_overlay_sysctl+0x428/0xc68
(XEN)    [<00000a00002707f8>] arch_do_sysctl+0x1c/0x2c
(XEN)    [<00000a0000230b40>] do_sysctl+0x96c/0x9ec
(XEN)    [<00000a0000271e08>] traps.c#do_trap_hypercall+0x1e8/0x288
(XEN)    [<00000a0000273490>] do_trap_guest_sync+0x448/0x63c
(XEN)    [<00000a000025c480>] entry.o#guest_sync_slowpath+0xa8/0xd8
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Assertion 'rw_is_locked(&dt_host_lock)' failed at 
drivers/passthrough/device_tree.c:146
(XEN) ****************************************

This is because iommu_remove_dt_device() is called without taking the
dt_host_lock. dt_host_lock is meant to ensure that the DT node will not
disappear behind back. So fix the issue by taking the lock as soon as getting
hold of overlay_node.

Fixes: 7e5c4a8b86f1 ("xen/arm: Implement device tree node removal 
functionalities")
Signed-off-by: Henry Wang <xin.wang2@xxxxxxx>
---
v2:
- Take the lock as soon as getting hold of overlay_node.
v1.1:
- Move the unlock position before the check of rc.
---
 xen/common/dt-overlay.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/common/dt-overlay.c b/xen/common/dt-overlay.c
index 1b197381f6..25d15cbcb1 100644
--- a/xen/common/dt-overlay.c
+++ b/xen/common/dt-overlay.c
@@ -429,6 +429,8 @@ static int remove_nodes(const struct overlay_track *tracker)
         if ( overlay_node == NULL )
             return -EINVAL;
 
+        write_lock(&dt_host_lock);
+
         rc = remove_descendant_nodes_resources(overlay_node);
         if ( rc )
             return rc;
@@ -439,8 +441,6 @@ static int remove_nodes(const struct overlay_track *tracker)
 
         dt_dprintk("Removing node: %s\n", overlay_node->full_name);
 
-        write_lock(&dt_host_lock);
-
         rc = dt_overlay_remove_node(overlay_node);
         if ( rc )
         {
-- 
2.34.1