On Thu, 04 Sep 2025 11:37:35 +0800
Menglong Dong [off-list ref] wrote:

On 2025/9/3 12:22 Herbert Xu [off-list ref] write:

quoted

On Tue, Sep 02, 2025 at 05:50:32PM +0800, menglong.dong@linux.dev wrote:

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On 2025/9/2 17:17 Herbert Xu [off-list ref] write:

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Menglong Dong [off-list ref] wrote:

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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)

Actually this isn't quite right.  I know that it is a false-positive
so that it's actually safe, but if you're going to mark it with
rcu_read_lock, it should cover both the lookup as well as the
dereference which happens in the loop rhl_for_each_entry_rcu.

Yeah, I understand. The rcu_read_lock() here is totally used to
suppress the suspicious rcu usage warning, not for the protection.
So I used it just for the rhltable_lookup() to reduce the impact.
Maybe I should add some comment for it.

My point is that after a lookup you will be doing some sort of a
dereference on the RCU pointer.  That would cause exactly the same
splat that rhltable_lookup itself generated.

For example, rhl_for_each_entry_rcu should have created the same
warning, but it doesn't because for some reason it is using
rcu_dereference_raw.  I'll need to dig up the history of this
to see if there is a good reason for it to not warn.

Yeah, I understand what you mean. I noticed this, and that's why
I added the rcu_read_lock() for rhashtable_lookup() only.

Maybe it is to obtain better performance? Just guess ;)
And hlist_for_each_entry_rcu() also uses rcu_dereference_raw().

Hi Menglong, if you update the patch to use guard(rcu)() because
head is used repeatedly in fprobe_entry(), I can replace it.

Thank you,

Thanks!
Menglong Dong

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Cheers,
-- 
Email: Herbert Xu [off-list ref]
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

-- 
Masami Hiramatsu (Google) [off-list ref]