Merge: rcu/nocb: Add Safe checks for access offloaded rdp

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-10/-/merge_requests/1324

JIRA: https://issues.redhat.com/browse/RHEL-69516

    commit 36f8e30
    Author: Zqiang <qiang.zhang1211@gmail.com>
    Date:   Wed May 7 19:26:04 2025 +0800

        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>

Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>

Approved-by: Crystal Wood <crwood@redhat.com>
Approved-by: Rafael Aquini <raquini@redhat.com>
Approved-by: Waiman Long <longman@redhat.com>
Approved-by: Phil Auld <pauld@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: Scott Weaver <scweaver@redhat.com>

Author: swvrrh

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"