linux-kernel - Re: [PATCH] kprobes: Fix to delay the kprobes jump optimization

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Date:   Fri, 19 Feb 2021 11:49:58 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Michal Hocko <mhocko@...e.com>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH] kprobes: Fix to delay the kprobes jump optimization

On Fri, Feb 19, 2021 at 09:17:55AM +0100, Sebastian Andrzej Siewior wrote:
> On 2021-02-18 07:15:54 [-0800], Paul E. McKenney wrote:
> > Thank you, but the original report of a problem was from Sebastian
> > and the connection to softirq was Uladzislau.  So could you please
> > add these before (or even in place of) my Reported-by?
> > 
> > Reported-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > Reported-by: Uladzislau Rezki <urezki@...il.com>
> > 
> > Other than that, looks good!
> 
> Perfect. I'm kind of lost here, nevertheless ;) Does this mean that the
> RCU selftest can now be delayed?
> 
If above fix works, we can initialize rcu_init_tasks_generic() from the
core_initcall() including selftst. It means that such initialization can
be done later:

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 5cc6deaa5df2..ae7d0cdfa9bd 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -88,12 +88,6 @@ void rcu_sched_clock_irq(int user);
 void rcu_report_dead(unsigned int cpu);
 void rcutree_migrate_callbacks(int cpu);
 
-#ifdef CONFIG_TASKS_RCU_GENERIC
-void rcu_init_tasks_generic(void);
-#else
-static inline void rcu_init_tasks_generic(void) { }
-#endif
-
 #ifdef CONFIG_RCU_STALL_COMMON
 void rcu_sysrq_start(void);
 void rcu_sysrq_end(void);
diff --git a/init/main.c b/init/main.c
index c68d784376ca..3024c4db17a9 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1512,7 +1512,6 @@ static noinline void __init kernel_init_freeable(void)
 
 	init_mm_internals();
 
-	rcu_init_tasks_generic();
 	do_pre_smp_initcalls();
 	lockup_detector_init();
 
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 17c8ebe131af..2797f9a042f4 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -966,11 +966,6 @@ static void rcu_tasks_trace_pregp_step(void)
 static void rcu_tasks_trace_pertask(struct task_struct *t,
 				    struct list_head *hop)
 {
-	// During early boot when there is only the one boot CPU, there
-	// is no idle task for the other CPUs. Just return.
-	if (unlikely(t == NULL))
-		return;
-
 	WRITE_ONCE(t->trc_reader_special.b.need_qs, false);
 	WRITE_ONCE(t->trc_reader_checked, false);
 	t->trc_ipi_to_cpu = -1;
@@ -1300,7 +1295,7 @@ late_initcall(rcu_tasks_verify_self_tests);
 static void rcu_tasks_initiate_self_tests(void) { }
 #endif /* #else #ifdef CONFIG_PROVE_RCU */
 
-void __init rcu_init_tasks_generic(void)
+static void __init rcu_init_tasks_generic(void)
 {
 #ifdef CONFIG_TASKS_RCU
 	rcu_spawn_tasks_kthread();
@@ -1318,6 +1313,7 @@ void __init rcu_init_tasks_generic(void)
 	rcu_tasks_initiate_self_tests();
 }
 
+core_initcall(rcu_init_tasks_generic);
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
 static inline void rcu_tasks_bootup_oddness(void) {}
 void show_rcu_tasks_gp_kthreads(void) {}


--
Vlad Rezki