Re: [PATCH] tracing: fprobe: Fix RCU warning message in list traversal

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 9 May 2025 16:30:08 +0200
Matthieu Baerts <matttbe@kernel.org> wrote:

> Hi Breno,
> 
> On 10/04/2025 14:22, Breno Leitao wrote:
> > When CONFIG_PROVE_RCU_LIST is enabled, fprobe triggers the following
> > warning:
> > 
> >     WARNING: suspicious RCU usage
> >     kernel/trace/fprobe.c:457 RCU-list traversed in non-reader section!!
> > 
> >     other info that might help us debug this:
> > 	#1: ffffffff863c4e08 (fprobe_mutex){+.+.}-{4:4}, at: fprobe_module_callback+0x7b/0x8c0
> > 
> >     Call Trace:
> > 	fprobe_module_callback
> > 	notifier_call_chain
> > 	blocking_notifier_call_chain
> > 
> > This warning occurs because fprobe_remove_node_in_module() traverses an
> > RCU list using RCU primitives without holding an RCU read lock. However,
> > the function is only called from fprobe_module_callback(), which holds
> > the fprobe_mutex lock that provides sufficient protection for safely
> > traversing the list.
> > 
> > Fix the warning by specifying the locking design to the
> > CONFIG_PROVE_RCU_LIST mechanism. Add the lockdep_is_held() argument to
> > hlist_for_each_entry_rcu() to inform the RCU checker that fprobe_mutex
> > provides the required protection.
> 
> @Breno: thank you for the patch, I have it applied for a while on a
> branch dedicated to our CI to avoid false positive reports.
> 
> Tested-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>

Oops, I missed this patch.

Let me pick it up and send it urgently.

Thank you,

> 
> 
> Hopefully this fix can be sent to Linus before the v6.15 release :)
> 
> Cheers,
> Matt
> -- 
> Sponsored by the NGI0 Core fund.
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>