On Mon, 1 Sep 2025 08:00:15 -0700 "Paul E. McKenney" <[email protected]> wrote:
> On Mon, Sep 01, 2025 at 05:06:55PM +0900, Masami Hiramatsu wrote: > > On Fri, 29 Aug 2025 04:11:02 -0700 > > "Paul E. McKenney" <[email protected]> wrote: > > > > > On Thu, Aug 28, 2025 at 10:23:57PM -0400, Steven Rostedt wrote: > > > > 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(). > > > > > > > > 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. > > > > That is similar to the kprobes, which also allows accessing in > > rcu critical section or under mutex. > > > > > 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. > > > > What about changing it alloing it with preempt disabled flag? > > I am not sure that "it" that you are proposing changing. ;-) Sorry, Ii meant the rcu_dereference_check(). > > However, another option for the the above rcu_dereference_check() to > become something like this: > > rcu_dereference_check(p, lockdep_rht_mutex_is_held(ht) || > rcu_read_lock_any_held()); > > This would be happy with any RCU reader, including rcu_read_lock(), > preempt_disable(), local_irq_disable(), local_bh_disable(), and various > handler contexts. One downside is that this would *always* be happy in > a kernel built with CONFIG_PREEMPT_{NONE,VOLUNTARY}=y. Ah, indeed. This means that we lose the ability to explicitly check whether the rcu pointer is in a critical section on that kernel. > > If this is happening often enough, it would be easy for me to create an > rcu_dereference_all_check() that allows all forms of vanilla RCU readers > (but not, for example, SRCU readers), but with only two use cases, > it is not clear to me that this is an overall win. OK, I think this discussion is important for the patch from Menglong [1] [1] https://lore.kernel.org/all/[email protected]/ because this does not make an rcu critical section while using `head` but it works because fprobe_entry() runs under preempt_disable(). Is it better to use `guard(rcu)()` instead of rcu_read_lock() so that it explicitly secure the `head` usage? I just wonder if there is any downside to extend rcu_read_lock() area (still in the same preempt_disable section). Thank you, > > Or am I missing a turn in here somewhere? > > Thanx, Paul > > > Thank you, > > > > > > > > Thanx, Paul > > > > > > > -- 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) > > > > > > > > > > -- > > Masami Hiramatsu (Google) <[email protected]> -- Masami Hiramatsu (Google) <[email protected]>
