linux-kernel - Re: [PATCH v7 5/6] kernfs: Use RCU to access kernfs

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Message-ID: <20250210084331.IJB3qKdl@linutronix.de>
Date: Mon, 10 Feb 2025 09:43:31 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Michal Koutný <mkoutny@...e.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Hillf Danton <hdanton@...a.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Marco Elver <elver@...gle.com>, Tejun Heo <tj@...nel.org>,
	tglx@...utronix.de, Andrii Nakryiko <andrii@...nel.org>,
	Eduard Zingerman <eddyz87@...il.com>,
	Mykola Lysenko <mykolal@...com>, bpf@...r.kernel.org
Subject: Re: [PATCH v7 5/6] kernfs: Use RCU to access kernfs_node::parent.

On 2025-02-03 14:50:22 [+0100], To cgroups@...r.kernel.org wrote:
> kernfs_rename_lock is used to obtain stable kernfs_node::{name|parent}
> pointer. This is a preparation to access kernfs_node::parent under RCU
> and ensure that the pointer remains stable under the RCU lifetime
> guarantees.
…

The robot complained that the selftests for bpf broke. As it turns out,
the tests access kernfs_node::{parent|name} and after the rename
::parent is gone so it does not compile.
If there are no objections, I would merge this into 5/6 and repost.
"test_progs -a test_profiler" passes.

diff --git a/tools/testing/selftests/bpf/progs/profiler.inc.h b/tools/testing/selftests/bpf/progs/profiler.inc.h
index 8bd1ebd7d6afd..a4f518ee5f4de 100644
--- a/tools/testing/selftests/bpf/progs/profiler.inc.h
+++ b/tools/testing/selftests/bpf/progs/profiler.inc.h
@@ -223,7 +223,7 @@ static INLINE void* read_full_cgroup_path(struct kernfs_node* cgroup_node,
 		if (bpf_cmp_likely(filepart_length, <=, MAX_PATH)) {
 			payload += filepart_length;
 		}
-		cgroup_node = BPF_CORE_READ(cgroup_node, parent);
+		cgroup_node = BPF_CORE_READ(cgroup_node, __parent);
 	}
 	return payload;
 }
@@ -300,6 +300,7 @@ static INLINE void* populate_cgroup_info(struct cgroup_data_t* cgroup_data,
 	cgroup_data->cgroup_proc_length = 0;
 	cgroup_data->cgroup_full_length = 0;
 
+	bpf_rcu_read_lock();
 	size_t cgroup_root_length =
 		bpf_probe_read_kernel_str(payload, MAX_PATH,
 					  BPF_CORE_READ(root_kernfs, name));
@@ -323,6 +324,7 @@ static INLINE void* populate_cgroup_info(struct cgroup_data_t* cgroup_data,
 		cgroup_data->cgroup_full_length = payload_end_pos - payload;
 		payload = payload_end_pos;
 	}
+	bpf_rcu_read_unlock();
 
 	return (void*)payload;
 }
-- 
2.47.2

Sebastian