linux-kernel - Re: [PATCH] kernel: trace_syscalls: Replace rcu_assign_pointer() with RCU_INIT

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Date:	Fri, 22 Aug 2014 11:37:58 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andreea-Cristina Bernat <bernat.ada@...il.com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] kernel: trace_syscalls: Replace rcu_assign_pointer()
 with RCU_INIT_POINTER()

On Fri, 22 Aug 2014 17:28:22 +0300
Andreea-Cristina Bernat <bernat.ada@...il.com> wrote:

> The uses of "rcu_assign_pointer()" are NULLing out the pointers.
> According to RCU_INIT_POINTER()'s block comment:
> "1.   This use of RCU_INIT_POINTER() is NULLing out the pointer"
> it is better to use it instead of rcu_assign_pointer() because it has a
> smaller overhead.
> 
> The following Coccinelle semantic patch was used:
> @@
> @@
> 
> - rcu_assign_pointer
> + RCU_INIT_POINTER
>   (..., NULL)

I guess I can add this. It's a very slow path thus it isn't critical.

Although, I hate the name. Perhaps we should add another macro called
RCU_CLEAR_POINTER() or something that just nulls it. That way it
documents the use. To me, INIT means the pointer is being initialized,
where in reality it's just being cleared. I guess one could argue that
the pointer is being "re-initialized".

Thanks!

-- Steve

> 
> Signed-off-by: Andreea-Cristina Bernat <bernat.ada@...il.com>
> ---
>  kernel/trace/trace_syscalls.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
> index 759d5e0..4dc8b79 100644
> --- a/kernel/trace/trace_syscalls.c
> +++ b/kernel/trace/trace_syscalls.c
> @@ -425,7 +425,7 @@ static void unreg_event_syscall_enter(struct ftrace_event_file *file,
>  		return;
>  	mutex_lock(&syscall_trace_lock);
>  	tr->sys_refcount_enter--;
> -	rcu_assign_pointer(tr->enter_syscall_files[num], NULL);
> +	RCU_INIT_POINTER(tr->enter_syscall_files[num], NULL);
>  	if (!tr->sys_refcount_enter)
>  		unregister_trace_sys_enter(ftrace_syscall_enter, tr);
>  	mutex_unlock(&syscall_trace_lock);
> @@ -463,7 +463,7 @@ static void unreg_event_syscall_exit(struct ftrace_event_file *file,
>  		return;
>  	mutex_lock(&syscall_trace_lock);
>  	tr->sys_refcount_exit--;
> -	rcu_assign_pointer(tr->exit_syscall_files[num], NULL);
> +	RCU_INIT_POINTER(tr->exit_syscall_files[num], NULL);
>  	if (!tr->sys_refcount_exit)
>  		unregister_trace_sys_exit(ftrace_syscall_exit, tr);
>  	mutex_unlock(&syscall_trace_lock);

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