rcu/nocb: Add Safe checks for access offloaded rdp

For built with CONFIG_PROVE_RCU=y and CONFIG_PREEMPT_RT=y kernels,
Disable BH does not change the SOFTIRQ corresponding bits in
preempt_count(), but change current->softirq_disable_cnt, 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 softirq_count() check for
Preempt-RT kernels.

Suggested-by: Joel Fernandes <joelagnelf@nvidia.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 3c0bbbbb686fe2b92b3560cb0f12e7add6505636..0b0f56f6abc85307804e60396c8a7df8e5468a5a 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -29,7 +29,7 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
                  (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
                  lockdep_is_held(&rdp->nocb_lock) ||
                  lockdep_is_held(&rcu_state.nocb_mutex) ||
-                 (!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) &&
+                 ((!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) || softirq_count()) &&
                   rdp == this_cpu_ptr(&rcu_data)) ||
                  rcu_current_is_nocb_kthread(rdp)),
                "Unsafe read of RCU_NOCB offloaded state"