[PATCH] cpumask: fix spurious cpumask_of_node() on non-NUMA multi-node configs

[Tejun Heo]

When !CONFIG_NUMA, cpumask_of_node(@node) equals cpu_online_mask
regardless of @node.  The assumption seems that if !NUMA, there
shouldn't be more than one node and thus reporting cpu_online_mask
regardless of @node is correct.  However, that assumption was broken
years ago to support CONFIG_DISCONTIGMEM and whether a system has
multiple nodes or not is separately controlled by
CONFIG_NEED_MULTIPLE_NODES.

This means that, on a system with !CONFIG_NUMA &&
CONFIG_NEED_MULTIPLE_NODES, cpumask_of_node() will report
cpu_online_mask for all possible nodes, indicating that the CPUs are
associated with multiple nodes which is an impossible configuration.

This bug has been around forever but doesn't look like it has caused
any noticeable symptoms.  However, it triggers a WARN recently added
to workqueue to verify NUMA affinity configuration.

Fix it by reporting empty cpumask on non-zero nodes if !NUMA.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-and-tested-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 include/asm-generic/topology.h |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/topology.h b/include/asm-generic/topology.h
index fc824e2..5d2add1 100644
--- a/include/asm-generic/topology.h
+++ b/include/asm-generic/topology.h
@@ -48,7 +48,11 @@
 #define parent_node(node)	((void)(node),0)
 #endif
 #ifndef cpumask_of_node
-#define cpumask_of_node(node)	((void)node, cpu_online_mask)
+  #ifdef CONFIG_NEED_MULTIPLE_NODES
+    #define cpumask_of_node(node)	((node) == 0 ? cpu_online_mask : cpu_none_mask)
+  #else
+    #define cpumask_of_node(node)	((void)node, cpu_online_mask)
+  #endif
 #endif
 #ifndef pcibus_to_node
 #define pcibus_to_node(bus)	((void)(bus), -1)