From: Zqiang Date: Wed, 7 May 2025 11:26:04 +0000 (+0800) Subject: rcu/nocb: Add Safe checks for access offloaded rdp X-Git-Tag: v6.16-rc1~204^2~1 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=36f8e3087562bc3f55f584fb42d105dfdf6686f4;p=thirdparty%2Fkernel%2Flinux.git 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] [ 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] 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 Reviewed-by: Frederic Weisbecker Signed-off-by: Zqiang Signed-off-by: Joel Fernandes --- diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 3c0bbbbb686fe..0b0f56f6abc85 100644 --- 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"