Re: [PATCH v3 20/28] powerpc/ftrace: Use RCU in all users of __module_text_address().

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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@csgroup.eu>
> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Naveen N Rao <naveen@kernel.org>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: linux-trace-kernel@vger.kernel.org
> Cc: linuxppc-dev@lists.ozlabs.org
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.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@linux.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);
>