[PATCH linux-next][RFC] powerpc: protect cpu offlining by RCU offline lock

[Zhouyi Zhou]

From: Zhouyi Zhou <zhouzhouyi at gmail.com>

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 <zhouzhouyi at gmail.com>
---
Dear PPC and RCU developers

I found this bug when trying to do rcutorture tests in ppc VM of
Open Source Lab of Oregon State University.

console.log report following bug:
[   37.635545][    T0] WARNING: suspicious RCU usage^M
[   37.636409][    T0] 6.0.0-rc4-next-20220907-dirty #8 Not tainted^M
[   37.637575][    T0] -----------------------------^M
[   37.638306][    T0] kernel/locking/lockdep.c:3723 RCU-list traversed in non-reader section!!^M
[   37.639651][    T0] ^M
[   37.639651][    T0] other info that might help us debug this:^M
[   37.639651][    T0] ^M
[   37.641381][    T0] ^M
[   37.641381][    T0] RCU used illegally from offline CPU!^M
[   37.641381][    T0] rcu_scheduler_active = 2, debug_locks = 1^M
[   37.667170][    T0] no locks held by swapper/6/0.^M
[   37.668328][    T0] ^M
[   37.668328][    T0] stack backtrace:^M
[   37.669995][    T0] CPU: 6 PID: 0 Comm: swapper/6 Not tainted 6.0.0-rc4-next-20220907-dirty #8^M
[   37.672777][    T0] Call Trace:^M
[   37.673729][    T0] [c000000004653920] [c00000000097f9b4] dump_stack_lvl+0x98/0xe0 (unreliable)^M
[   37.678579][    T0] [c000000004653960] [c0000000001f2eb8] lockdep_rcu_suspicious+0x148/0x16c^M
[   37.680425][    T0] [c0000000046539f0] [c0000000001ed9b4] __lock_acquire+0x10f4/0x26e0^M
[   37.682450][    T0] [c000000004653b30] [c0000000001efc2c] lock_acquire+0x12c/0x420^M
[   37.684113][    T0] [c000000004653c20] [c0000000010d704c] _raw_spin_lock_irqsave+0x6c/0xc0^M
[   37.686154][    T0] [c000000004653c60] [c0000000000c7b4c] xive_spapr_put_ipi+0xcc/0x150^M
[   37.687879][    T0] [c000000004653ca0] [c0000000010c72a8] xive_cleanup_cpu_ipi+0xc8/0xf0^M
[   37.689856][    T0] [c000000004653cf0] [c0000000010c7370] xive_teardown_cpu+0xa0/0xf0^M
[   37.691877][    T0] [c000000004653d30] [c0000000000fba5c] pseries_cpu_offline_self+0x5c/0x100^M
[   37.693882][    T0] [c000000004653da0] [c00000000005d2c4] arch_cpu_idle_dead+0x44/0x60^M
[   37.695739][    T0] [c000000004653dc0] [c0000000001c740c] do_idle+0x16c/0x3d0^M
[   37.697536][    T0] [c000000004653e70] [c0000000001c7a1c] cpu_startup_entry+0x3c/0x40^M
[   37.699694][    T0] [c000000004653ea0] [c00000000005ca20] start_secondary+0x6c0/0xb50^M
[   37.701742][    T0] [c000000004653f90] [c00000000000d054] start_secondary_prolog+0x10/0x14^M


I am a beginner, hope I can be of some beneficial to the community ;-)

Thanks
Zhouyi
--
 arch/powerpc/platforms/pseries/hotplug-cpu.c |  5 ++++-
 include/linux/rcupdate.h                     |  3 ++-
 kernel/rcu/tree.c                            | 10 ++++++++++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index 0f8cd8b06432..ddf66a253c70 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/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();
 
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 63d2e6a60ad7..d857955a02ba 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -1034,5 +1034,6 @@ 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 */
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6bb8e72bc815..3282725f1054 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4796,6 +4796,16 @@ 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"
-- 
2.34.1