powerpc: protect cpu offlining by RCU offline lock

During the cpu offlining, the sub functions of xive_teardown_cpu will
call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will
travel RCU protected list, so "WARNING: suspicious RCU usage" will be
triggered.

Try to protect cpu offlining by RCU offline lock.

Tested on PPC VM of Open Source Lab of Oregon State University.
(Each round of tests takes about 19 hours to finish)
Test results show that although "WARNING: suspicious RCU usage" has gone,
but there are more "BUG: soft lockup" reports than the original kernel
(10 vs 6), so I add a [RFC] to my subject line.

Signed-off-by: Zhouyi Zhou <[email protected]>

Author: zhouzhouyi-hub

arch/powerpc/platforms/pseries/hotplug-cpu.c

@@ -64,11 +64,14 @@ static void pseries_cpu_offline_self(void)
 
 	local_irq_disable();
 	idle_task_exit();
+
+	/* Because the cpu is now offline, let rcu know that */
+	rcu_state_ofl_lock();
 	if (xive_enabled())
 		xive_teardown_cpu();
 	else
 		xics_teardown_cpu();
-
+	rcu_state_ofl_unlock();
 	unregister_slb_shadow(hwcpu);
 	rtas_stop_self();
 

include/linux/rcupdate.h

@@ -1055,5 +1055,6 @@ rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
 /* kernel/ksysfs.c definitions */
 extern int rcu_expedited;
 extern int rcu_normal;
-
+void rcu_state_ofl_lock(void);
+void rcu_state_ofl_unlock(void);
 #endif /* __LINUX_RCUPDATE_H */

kernel/rcu/tree.c

@@ -4815,6 +4815,16 @@ void __init rcu_init(void)
 		(void)start_poll_synchronize_rcu_expedited();
 }
 
+void rcu_state_ofl_lock(void)
+{
+	arch_spin_lock(&rcu_state.ofl_lock);
+}
+
+void rcu_state_ofl_unlock(void)
+{
+	arch_spin_unlock(&rcu_state.ofl_lock);
+}
+
 #include "tree_stall.h"
 #include "tree_exp.h"
 #include "tree_nocb.h"