[PATCH] rcu/nocb: Add Safe checks for access offloaded rdp

[Zqiang <qiang.zhang1211@gmail.com>]

For Preempt-RT kernel, when enable CONFIG_PROVE_RCU Kconfig,
disable local bh in rcuc kthreads will not affect preempt_count(),
this resulted in the following splat:

WARNING: suspicious RCU usage
kernel/rcu/tree_plugin.h:36 Unsafe read of RCU_NOCB offloaded state!
stack backtrace:
CPU: 0 UID: 0 PID: 22 Comm: rcuc/0
Call Trace:
[    0.407907]  <TASK>
[    0.407910]  dump_stack_lvl+0xbb/0xd0
[    0.407917]  dump_stack+0x14/0x20
[    0.407920]  lockdep_rcu_suspicious+0x133/0x210
[    0.407932]  rcu_rdp_is_offloaded+0x1c3/0x270
[    0.407939]  rcu_core+0x471/0x900
[    0.407942]  ? lockdep_hardirqs_on+0xd5/0x160
[    0.407954]  rcu_cpu_kthread+0x25f/0x870
[    0.407959]  ? __pfx_rcu_cpu_kthread+0x10/0x10
[    0.407966]  smpboot_thread_fn+0x34c/0xa50
[    0.407970]  ? trace_preempt_on+0x54/0x120
[    0.407977]  ? __pfx_smpboot_thread_fn+0x10/0x10
[    0.407982]  kthread+0x40e/0x840
[    0.407990]  ? __pfx_kthread+0x10/0x10
[    0.407994]  ? rt_spin_unlock+0x4e/0xb0
[    0.407997]  ? rt_spin_unlock+0x4e/0xb0
[    0.408000]  ? __pfx_kthread+0x10/0x10
[    0.408006]  ? __pfx_kthread+0x10/0x10
[    0.408011]  ret_from_fork+0x40/0x70
[    0.408013]  ? __pfx_kthread+0x10/0x10
[    0.408018]  ret_from_fork_asm+0x1a/0x30
[    0.408042]  </TASK>

Currently, triggering an rdp offloaded state change need the
corresponding rdp's CPU goes offline, and at this time the rcuc
kthreads has already in parking state. this means the corresponding
rcuc kthreads can safely read offloaded state of rdp while it's
corresponding cpu is online.

This commit therefore add rdp->rcu_cpu_kthread_task check for
Preempt-RT kernels.

Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
---
 kernel/rcu/tree_plugin.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 003e549f6514..fe728eded36e 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -31,7 +31,9 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
 		  lockdep_is_held(&rcu_state.nocb_mutex) ||
 		  (!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) &&
 		   rdp == this_cpu_ptr(&rcu_data)) ||
-		  rcu_current_is_nocb_kthread(rdp)),
+		  rcu_current_is_nocb_kthread(rdp) ||
+		  (IS_ENABLED(CONFIG_PREEMPT_RT) &&
+		   current == rdp->rcu_cpu_kthread_task)),
 		"Unsafe read of RCU_NOCB offloaded state"
 	);
 
-- 
2.17.1