linux-kernel - [PATCH] docs: fix bug in "Krefs and RCU" example

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Date: Tue, 19 Mar 2024 10:26:45 +0300
From: Dmitry Mastykin <dmastykin@...ralinux.ru>
To: corbet@....net,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	willy@...radead.org
Cc: mastichi@...il.com,
	Dmitry Mastykin <dmastykin@...ralinux.ru>
Subject: [PATCH] docs: fix bug in "Krefs and RCU" example

There is a problem in "Krefs and RCU" example, that may cause a crash.
If list_del_rcu() will be called between these lines, and list will become
empty, then q.next will not point to a valid struct my_data.
entry->refcount will also be invalid.
Instead, q.next must be read first, and then compared with q to check
list's emptiness, like in list_first_or_null_rcu() macro.

Signed-off-by: Dmitry Mastykin <dmastykin@...ralinux.ru>
---
 Documentation/core-api/kref.rst | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/Documentation/core-api/kref.rst b/Documentation/core-api/kref.rst
index c61eea6f1bf2..022bb1d1b968 100644
--- a/Documentation/core-api/kref.rst
+++ b/Documentation/core-api/kref.rst
@@ -290,13 +290,11 @@ locking for lookups in the above example::
 
 	static struct my_data *get_entry_rcu()
 	{
-		struct my_data *entry = NULL;
+		struct my_data *entry;
 		rcu_read_lock();
-		if (!list_empty(&q)) {
-			entry = container_of(q.next, struct my_data, link);
-			if (!kref_get_unless_zero(&entry->refcount))
-				entry = NULL;
-		}
+		entry = list_first_or_null_rcu(&q, struct my_data, link);
+		if (entry && !kref_get_unless_zero(&entry->refcount))
+			entry = NULL;
 		rcu_read_unlock();
 		return entry;
 	}
-- 
2.30.2