Re: [PATCH v4 11/27] rcu,tracing: Create trace_rcu_{enter,exit}()

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Feb 21, 2020 at 02:34:27PM +0100, Peter Zijlstra wrote:
> To facilitate tracers that need RCU, add some helpers to wrap the
> magic required.
> 
> The problem is that we can call into tracers (trace events and
> function tracing) while RCU isn't watching and this can happen from
> any context, including NMI.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> ---
>  include/linux/rcupdate.h |   29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -175,6 +175,35 @@ do { \
>  #error "Unknown RCU implementation specified to kernel configuration"
>  #endif
>  
> +/**
> + * trace_rcu_enter - Force RCU to be active, for code that needs RCU readers
> + *
> + * Very similar to RCU_NONIDLE() above.
> + *
> + * Tracing can happen while RCU isn't active yet, for instance in the idle loop
> + * between rcu_idle_enter() and rcu_idle_exit(), or early in exception entry.
> + * RCU will happily ignore any read-side critical sections in this case.
> + *
> + * This function ensures that RCU is aware hereafter and the code can readily
> + * rely on RCU read-side critical sections working as expected.
> + *
> + * This function is NMI safe -- provided in_nmi() is correct and will nest up-to
> + * INT_MAX/2 times.
> + */
> +static inline int trace_rcu_enter(void)
> +{
> +	int state = !rcu_is_watching();
> +	if (state)
> +		rcu_irq_enter_irqsave();
> +	return state;
> +}
> +
> +static inline void trace_rcu_exit(int state)
> +{
> +	if (state)
> +		rcu_irq_exit_irqsave();
> +}
> +
>  /*
>   * The init_rcu_head_on_stack() and destroy_rcu_head_on_stack() calls
>   * are needed for dynamic initialization and destruction of rcu_head

Massmi; afaict we also need the below. That is, when you stick an
optimized kprobe in a region RCU is not watching, nothing will make RCU
go.

---
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 9ad5e6b346f8..fa14918613da 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -370,6 +370,7 @@ static bool kprobes_allow_optimization;
  */
 void opt_pre_handler(struct kprobe *p, struct pt_regs *regs)
 {
+	int rcu_flags = trace_rcu_enter();
 	struct kprobe *kp;

 	list_for_each_entry_rcu(kp, &p->list, list) {
@@ -379,6 +380,7 @@ void opt_pre_handler(struct kprobe *p, struct pt_regs *regs)
 		}
 		reset_kprobe_instance();
 	}
+	trace_rcu_exit(rcu_flags);
 }
 NOKPROBE_SYMBOL(opt_pre_handler);