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

[Shrikanth Hegde]

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.le...@csgroup.eu>
Cc: Madhavan Srinivasan <ma...@linux.ibm.com>
Cc: Mark Rutland <mark.rutl...@arm.com>
Cc: Masami Hiramatsu <mhira...@kernel.org>
Cc: Michael Ellerman <m...@ellerman.id.au>
Cc: Naveen N Rao <nav...@kernel.org>
Cc: Nicholas Piggin <npig...@gmail.com>
Cc: Steven Rostedt <rost...@goodmis.org>
Cc: linux-trace-ker...@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Acked-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bige...@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 <sshe...@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);