linux-kernel - Re: [PATCH v3 20/28] powerpc/ftrace: Use RCU in all users of __module_text

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Message-ID: <671a14f1-55a1-4d80-ae5c-83f20bf3037c@linux.ibm.com>
Date: Mon, 20 Jan 2025 15:39:58 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Daniel Gomez <da.gomez@...sung.com>, Luis Chamberlain
 <mcgrof@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Pavlu <petr.pavlu@...e.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Thomas Gleixner
 <tglx@...utronix.de>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Mark Rutland <mark.rutland@....com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Naveen N Rao <naveen@...nel.org>, Nicholas Piggin <npiggin@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-trace-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 20/28] powerpc/ftrace: Use RCU in all users of
 __module_text_address().



On 1/8/25 14:34, Sebastian Andrzej Siewior wrote:
> __module_text_address() can be invoked within a RCU section, there is no
> requirement to have preemption disabled.
> 
> Replace the preempt_disable() section around __module_text_address()
> with RCU.
> 
> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
> Cc: Madhavan Srinivasan <maddy@...ux.ibm.com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Naveen N Rao <naveen@...nel.org>
> Cc: Nicholas Piggin <npiggin@...il.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: linux-trace-kernel@...r.kernel.org
> Cc: linuxppc-dev@...ts.ozlabs.org
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>   arch/powerpc/kernel/trace/ftrace.c       | 6 ++----
>   arch/powerpc/kernel/trace/ftrace_64_pg.c | 6 ++----
>   2 files changed, 4 insertions(+), 8 deletions(-)
> 

Ran ftrace (function graph) on preempt=full kernel with rcutorture while doing modprobe/rmmod.
rcutorture succeeded and didn't see any splats.
If there is any other method to test it out, please let me know.

So for powerpc bits:
Tested-by: Shrikanth Hegde <sshegde@...ux.ibm.com>

> diff --git a/arch/powerpc/kernel/trace/ftrace.c b/arch/powerpc/kernel/trace/ftrace.c
> index 5ccd791761e8f..558d7f4e4bea6 100644
> --- a/arch/powerpc/kernel/trace/ftrace.c
> +++ b/arch/powerpc/kernel/trace/ftrace.c
> @@ -115,10 +115,8 @@ static unsigned long ftrace_lookup_module_stub(unsigned long ip, unsigned long a
>   {
>   	struct module *mod = NULL;
>   
> -	preempt_disable();
> -	mod = __module_text_address(ip);
> -	preempt_enable();
> -
> +	scoped_guard(rcu)
> +		mod = __module_text_address(ip);
>   	if (!mod)
>   		pr_err("No module loaded at addr=%lx\n", ip);
>   
> diff --git a/arch/powerpc/kernel/trace/ftrace_64_pg.c b/arch/powerpc/kernel/trace/ftrace_64_pg.c
> index 98787376eb87c..531d40f10c8a1 100644
> --- a/arch/powerpc/kernel/trace/ftrace_64_pg.c
> +++ b/arch/powerpc/kernel/trace/ftrace_64_pg.c
> @@ -120,10 +120,8 @@ static struct module *ftrace_lookup_module(struct dyn_ftrace *rec)
>   {
>   	struct module *mod;
>   
> -	preempt_disable();
> -	mod = __module_text_address(rec->ip);
> -	preempt_enable();
> -
> +	scoped_guard(rcu)
> +		mod = __module_text_address(rec->ip);
>   	if (!mod)
>   		pr_err("No module loaded at addr=%lx\n", rec->ip);
>