linux-kernel - Re: [for-next][PATCH 4/4] ftrace: Add comment to why rcu_dereference

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Date:   Wed, 5 Feb 2020 06:33:49 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Amol Grover <frextrite@...il.com>
Subject: Re: [for-next][PATCH 4/4] ftrace: Add comment to why
 rcu_dereference_sched() is open coded


Paul and Joel,

Care to ack this patch (or complain about it ;-) ?

-- Steve


On Wed, 05 Feb 2020 05:49:33 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> Because the function graph tracer can execute in sections where RCU is not
> "watching", the rcu_dereference_sched() for the has needs to be open coded.
> This is fine because the RCU "flavor" of the ftrace hash is protected by
> its own RCU handling (it does its own little synchronization on every CPU
> and does not rely on RCU sched).
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>  kernel/trace/trace.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 022def96d307..8c52f5de9384 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -975,6 +975,11 @@ static inline int ftrace_graph_addr(struct ftrace_graph_ent *trace)
>  
>  	preempt_disable_notrace();
>  
> +	/*
> +	 * Have to open code "rcu_dereference_sched()" because the
> +	 * function graph tracer can be called when RCU is not
> +	 * "watching".
> +	 */
>  	hash = rcu_dereference_protected(ftrace_graph_hash, !preemptible());
>  
>  	if (ftrace_hash_empty(hash)) {
> @@ -1022,6 +1027,11 @@ static inline int ftrace_graph_notrace_addr(unsigned long addr)
>  
>  	preempt_disable_notrace();
>  
> +	/*
> +	 * Have to open code "rcu_dereference_sched()" because the
> +	 * function graph tracer can be called when RCU is not
> +	 * "watching".
> +	 */
>  	notrace_hash = rcu_dereference_protected(ftrace_graph_notrace_hash,
>  						 !preemptible());
>