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

[Antonio Quartulli]

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.
>
> Fixes: a3dc2983ca7b90 ("tracing: fprobe: Cleanup fprobe hash when module unloading")
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>

I just hit this issue and I verified that the patch actually solves it 
without causing side effects.

FWIW

Tested-by: Antonio Quartulli <antonio@xxxxxxxxxxxxx>

--
Antonio Quartulli

CEO and Co-Founder
Mandelbit Srl
https://www.mandelbit.com