On Fri, Aug 29, 2025 at 10:49:46AM +0800, [email protected] wrote:
> On 2025/8/29 10:23 Steven Rostedt <[email protected]> write:
> > On Fri, 29 Aug 2025 10:14:36 +0800
> > Menglong Dong <[email protected]> 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().
>
> It is needed when the lock debug configs are enabled.
>
> >
> > 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?
>
> Maybe we can do some adjustment do rcu_read_lock_held_common()
> like this:
>
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index c912b594ba98..280fa4d2fc79 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -114,6 +114,10 @@ static bool rcu_read_lock_held_common(bool *ret)
> *ret = false;
> return true;
> }
> + if (!preemptible()) {
> + *ret = true;
> + return true;
> + }
> return false;
> }
>
> @@ -123,7 +127,7 @@ int rcu_read_lock_sched_held(void)
>
> if (rcu_read_lock_held_common(&ret))
> return ret;
> - return lock_is_held(&rcu_sched_lock_map) || !preemptible();
> + return lock_is_held(&rcu_sched_lock_map);
> }
> EXPORT_SYMBOL(rcu_read_lock_sched_held);
> #endif
>
> I think it's a bad idea, as !preemptiable() has different semantic
> with rcu_read_lock() :(
Especially given the definition of preemptible() within kernels built
with CONFIG_PREEMPT_COUNT=n...
Thanx, Paul
> Thanks!
> Menglong Dong
>
> >
> > -- Steve
> >
> >
> > >
> > > Reported-by: kernel test robot <[email protected]>
> > > Closes: https://lore.kernel.org/oe-lkp/[email protected]
> > > Fixes: dfe0d675df82 ("tracing: fprobe: use rhltable for fprobe_ip_table")
> > > Signed-off-by: Menglong Dong <[email protected]>
> > > ---
> > > 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)
> >
> >
> >
>
>
>
>
