linux-kernel - Re: [PATCH] rcu,ftrace: Fix ftrace recursion

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Message-ID: <20200929143146.GT29330@paulmck-ThinkPad-P72>
Date:   Tue, 29 Sep 2020 07:31:46 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, kim.phillips@....com
Subject: Re: [PATCH] rcu,ftrace: Fix ftrace recursion

On Tue, Sep 29, 2020 at 01:33:40PM +0200, Peter Zijlstra wrote:
> 
> Kim reported that perf-ftrace made his box unhappy. It turns out that
> commit:
> 
>   ff5c4f5cad33 ("rcu/tree: Mark the idle relevant functions noinstr")
> 
> removed one too many notrace. Probably due to there not being a helpful
> comment.
> 
> Reinstate the notrace and add a comment to avoid loosing it again.

s/loosing/losing/, but otherwise:

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

But please let me know if you would prefer that I take it via -rcu.

						Thanx, Paul

> Fixes: ff5c4f5cad33 ("rcu/tree: Mark the idle relevant functions noinstr")
> Reported-by: Kim Phillips <kim.phillips@....com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/rcu/tree.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index ee5e595501e8..33020d84ec6b 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1098,8 +1098,11 @@ noinstr bool __rcu_is_watching(void)
>   * CPU can safely enter RCU read-side critical sections.  In other words,
>   * if the current CPU is not in its idle loop or is in an interrupt or
>   * NMI handler, return true.
> + *
> + * Must be notrace because __ftrace_ops_list_func() / ftrace_ops_assist_func()
> + * will call this (for every function) outside of recursion protection.
>   */
> -bool rcu_is_watching(void)
> +notrace bool rcu_is_watching(void)
>  {
>  	bool ret;
>