Re: [PATCH] tracing: Annotate ftrace_graph_hash pointer with __rcu

From: Joel Fernandes <joel@joelfernandes.org>
To: Amol Grover <frextrite@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org,
	linux-kernel-mentees@lists.linuxfoundation.org,
	Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
	"Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH] tracing: Annotate ftrace_graph_hash pointer with __rcu
Date: Mon, 3 Feb 2020 11:33:01 -0500	[thread overview]
Message-ID: <20200203163301.GB85781@google.com> (raw)
In-Reply-To: <20200201072703.17330-1-frextrite@gmail.com>

On Sat, Feb 01, 2020 at 12:57:04PM +0530, Amol Grover wrote:
> Fix following instances of sparse error
> kernel/trace/ftrace.c:5664:29: error: incompatible types in comparison
> kernel/trace/ftrace.c:5785:21: error: incompatible types in comparison
> kernel/trace/ftrace.c:5864:36: error: incompatible types in comparison
> kernel/trace/ftrace.c:5866:25: error: incompatible types in comparison
> 
> Use rcu_dereference_protected to access the __rcu annotated pointer.
> 
> Signed-off-by: Amol Grover <frextrite@gmail.com>
> ---
>  kernel/trace/ftrace.c | 2 +-
>  kernel/trace/trace.h  | 9 ++++++---
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 9bf1f2cd515e..959ded08dc13 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5596,7 +5596,7 @@ static const struct file_operations ftrace_notrace_fops = {
>  
>  static DEFINE_MUTEX(graph_lock);
>  
> -struct ftrace_hash *ftrace_graph_hash = EMPTY_HASH;
> +struct ftrace_hash __rcu *ftrace_graph_hash = EMPTY_HASH;
>  struct ftrace_hash *ftrace_graph_notrace_hash = EMPTY_HASH;
>  
>  enum graph_filter_type {
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 63bf60f79398..97dad3326020 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -950,22 +950,25 @@ extern void __trace_graph_return(struct trace_array *tr,
>  				 unsigned long flags, int pc);
>  
>  #ifdef CONFIG_DYNAMIC_FTRACE
> -extern struct ftrace_hash *ftrace_graph_hash;
> +extern struct ftrace_hash __rcu *ftrace_graph_hash;
>  extern struct ftrace_hash *ftrace_graph_notrace_hash;
>  
>  static inline int ftrace_graph_addr(struct ftrace_graph_ent *trace)
>  {
>  	unsigned long addr = trace->func;
>  	int ret = 0;
> +	struct ftrace_hash *hash;
>  
>  	preempt_disable_notrace();
>  
> -	if (ftrace_hash_empty(ftrace_graph_hash)) {
> +	hash = rcu_dereference_protected(ftrace_graph_hash, !preemptible());

I think you can use rcu_dereference_sched() here? That way no need to pass
!preemptible.

A preempt-disabled section is an RCU "sched flavor" section. Flavors are
consolidated in the backend, but in the front end the dereference API still
do have flavors.

thanks,

 - Joel

> +
> +	if (ftrace_hash_empty(hash)) {
>  		ret = 1;
>  		goto out;
>  	}
>  
> -	if (ftrace_lookup_ip(ftrace_graph_hash, addr)) {
> +	if (ftrace_lookup_ip(hash, addr)) {
>  
>  		/*
>  		 * This needs to be cleared on the return functions
> -- 
> 2.24.1
> 