Subject: Re: linux-next: build warning after merge of the tip tree

From: Peter Zijlstra <[email-masked]>
Date: Fri, 17 Sep 2021 10:23:48 +0200

On Fri, Sep 17, 2021 at 11:58:59AM +1000, Stephen Rothwell wrote:
> Hi all,
> 
> After merging the tip tree, today's linux-next build (x86_64 allmodconfig)
> produced this warning:
> 
> vmlinux.o: warning: objtool: mce_setup()+0x22: call to memset() leaves .noinstr.text section
> vmlinux.o: warning: objtool: do_machine_check()+0x51: call to mce_gather_info() leaves .noinstr.text section

Those are pre-existing and Boris is lokoing into them, these however,
are new:

> vmlinux.o: warning: objtool: rcu_dynticks_eqs_enter()+0x0: call to rcu_dynticks_task_trace_enter() leaves .noinstr.text section
> vmlinux.o: warning: objtool: rcu_dynticks_eqs_exit()+0xe: call to rcu_dynticks_task_trace_exit() leaves .noinstr.text section

These are from 7d0c9c50c5a1 ("rcu-tasks: Avoid IPIing userspace/idle
tasks if kernel is so built").

The below seems to help.

> vmlinux.o: warning: objtool: rcu_nmi_enter()+0x36: call to __kasan_check_read() leaves .noinstr.text section

That's 2be57f732889 ("rcu: Weaken ->dynticks accesses and updates"),
doing:

-       return !(arch_atomic_read(&this_cpu_ptr(&rcu_data)->dynticks) & 0x1);
+       return !(atomic_read(this_cpu_ptr(&rcu_data.dynticks)) & 0x1);

and causing instrumentation to be inserted where none is acceptable.
Flipping that back to arch_atomic_read() makes it go away as expected.

---
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 1a6fdb03d0a5..5199559fbbf0 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1479,7 +1479,7 @@ static void rcu_bind_gp_kthread(void)
 }
 
 /* Record the current task on dyntick-idle entry. */
-static void noinstr rcu_dynticks_task_enter(void)
+static __always_inline void rcu_dynticks_task_enter(void)
 {
 #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
 	WRITE_ONCE(current->rcu_tasks_idle_cpu, smp_processor_id());
@@ -1487,7 +1487,7 @@ static void noinstr rcu_dynticks_task_enter(void)
 }
 
 /* Record no current task on dyntick-idle exit. */
-static void noinstr rcu_dynticks_task_exit(void)
+static __always_inline void rcu_dynticks_task_exit(void)
 {
 #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
 	WRITE_ONCE(current->rcu_tasks_idle_cpu, -1);
@@ -1495,7 +1495,7 @@ static void noinstr rcu_dynticks_task_exit(void)
 }
 
 /* Turn on heavyweight RCU tasks trace readers on idle/user entry. */
-static void rcu_dynticks_task_trace_enter(void)
+static __always_inline void rcu_dynticks_task_trace_enter(void)
 {
 #ifdef CONFIG_TASKS_TRACE_RCU
 	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
@@ -1504,7 +1504,7 @@ static void rcu_dynticks_task_trace_enter(void)
 }
 
 /* Turn off heavyweight RCU tasks trace readers on idle/user exit. */
-static void rcu_dynticks_task_trace_exit(void)
+static __always_inline void rcu_dynticks_task_trace_exit(void)
 {
 #ifdef CONFIG_TASKS_TRACE_RCU
 	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))