powerpc: Only obtain cpu_hotplug_lock if called by rtasd

Calling arch_update_cpu_topology from a CPU hotplug state machine callback
hits a deadlock because the function tries to get a read lock on
cpu_hotplug_lock while the state machine still holds a write lock on it.

Since all callers of arch_update_cpu_topology except rtasd already hold
cpu_hotplug_lock, this patch changes the function to use
stop_machine_cpuslocked and creates a separate function for rtasd which
still tries to obtain the lock.

Michael Bringmann investigated the bug and provided a detailed analysis
of the deadlock on this previous RFC for an alternate solution:

Signed-off-by: Thiago Jung Bauermann <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Michael Ellerman <[email protected]>
Cc: John Allen <[email protected]>
Cc: Michael Bringmann <[email protected]>
Cc: Nathan Fontenot <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Link: https://patchwork.ozlabs.org/patch/771293/

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
index 8b3b46b7b0f2..a2d36b7703ae 100644
--- a/arch/powerpc/include/asm/topology.h
+++ b/arch/powerpc/include/asm/topology.h
@@ -43,6 +43,7 @@ extern void __init dump_numa_cpu_topology(void);
 
 extern int sysfs_add_device_to_node(struct device *dev, int nid);
 extern void sysfs_remove_device_from_node(struct device *dev, int nid);
+extern int numa_update_cpu_topology(bool cpus_locked);
 
 #else
 
@@ -57,6 +58,11 @@ static inline void sysfs_remove_device_from_node(struct device *dev,
 						int nid)
 {
 }
+
+static inline int numa_update_cpu_topology(bool cpus_locked)
+{
+	return 0;
+}
 #endif /* CONFIG_NUMA */
 
 #if defined(CONFIG_NUMA) && defined(CONFIG_PPC_SPLPAR)