On Thu, Aug 28, 2025 at 10:23:57PM -0400, Steven Rostedt wrote: > On Fri, 29 Aug 2025 10:14:36 +0800 > Menglong Dong <dongml2@xxxxxxxxxxxxxxx> wrote: > > > rcu_read_lock() is not needed in fprobe_entry, but rcu_dereference_check() > > is used in rhltable_lookup(), which causes suspicious RCU usage warning: > > > > WARNING: suspicious RCU usage > > 6.17.0-rc1-00001-gdfe0d675df82 #1 Tainted: G S > > ----------------------------- > > include/linux/rhashtable.h:602 suspicious rcu_dereference_check() usage! > > ...... > > stack backtrace: > > CPU: 1 UID: 0 PID: 4652 Comm: ftracetest Tainted: G S > > Tainted: [S]=CPU_OUT_OF_SPEC, [I]=FIRMWARE_WORKAROUND > > Hardware name: Dell Inc. OptiPlex 7040/0Y7WYT, BIOS 1.1.1 10/07/2015 > > Call Trace: > > <TASK> > > dump_stack_lvl+0x7c/0x90 > > lockdep_rcu_suspicious+0x14f/0x1c0 > > __rhashtable_lookup+0x1e0/0x260 > > ? __pfx_kernel_clone+0x10/0x10 > > fprobe_entry+0x9a/0x450 > > ? __lock_acquire+0x6b0/0xca0 > > ? find_held_lock+0x2b/0x80 > > ? __pfx_fprobe_entry+0x10/0x10 > > ? __pfx_kernel_clone+0x10/0x10 > > ? lock_acquire+0x14c/0x2d0 > > ? __might_fault+0x74/0xc0 > > function_graph_enter_regs+0x2a0/0x550 > > ? __do_sys_clone+0xb5/0x100 > > ? __pfx_function_graph_enter_regs+0x10/0x10 > > ? _copy_to_user+0x58/0x70 > > ? __pfx_kernel_clone+0x10/0x10 > > ? __x64_sys_rt_sigprocmask+0x114/0x180 > > ? __pfx___x64_sys_rt_sigprocmask+0x10/0x10 > > ? __pfx_kernel_clone+0x10/0x10 > > ftrace_graph_func+0x87/0xb0 > > > > Fix this by using rcu_read_lock() for rhltable_lookup(). Alternatively, we > > can use rcu_lock_acquire(&rcu_lock_map) here to obtain better performance. > > However, it's not a common usage :/ > > So this is needed even though it's called under preempt_disable(). > > Paul, do we need to add an rcu_read_lock() because the code in rht > (rhashtable) requires RCU read lock? > > I thought that rcu_read_lock() and preempt_disable() have been merged? Yes, preempt_disable() does indeed start an RCU read-side critical section, just as surely as rcu_read_lock() does. However, this is a lockdep check inside of __rhashtable_lookup(): rht_dereference_rcu(ht->tbl, ht) Which is defined as: rcu_dereference_check(p, lockdep_rht_mutex_is_held(ht)); This is explicitly telling lockdep that rcu_read_lock() is OK and holding ht->mutex is OK, but nothing else is. So an alternative way to fix this is to declare it to be a false positive, and then avoid that false positive by adding a check that preemption is disabled. Adding the rhashtable maintainers for their perspective. Thanx, Paul > -- Steve > > > > > > Reported-by: kernel test robot <oliver.sang@xxxxxxxxx> > > Closes: https://lore.kernel.org/oe-lkp/202508281655.54c87330-lkp@xxxxxxxxx > > Fixes: dfe0d675df82 ("tracing: fprobe: use rhltable for fprobe_ip_table") > > Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx> > > --- > > kernel/trace/fprobe.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c > > index fb127fa95f21..fece0f849c1c 100644 > > --- a/kernel/trace/fprobe.c > > +++ b/kernel/trace/fprobe.c > > @@ -269,7 +269,9 @@ static int fprobe_entry(struct ftrace_graph_ent *trace, struct fgraph_ops *gops, > > if (WARN_ON_ONCE(!fregs)) > > return 0; > > > > + rcu_read_lock(); > > head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params); > > + rcu_read_unlock(); > > reserved_words = 0; > > rhl_for_each_entry_rcu(node, pos, head, hlist) { > > if (node->addr != func) >
