linux-kernel - [PATCH rcu 5/7] doc: Update rcu_access_pointer() advice in rcu

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20220831180625.2692892-5-paulmck@kernel.org>
Date:   Wed, 31 Aug 2022 11:06:23 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 5/7] doc: Update rcu_access_pointer() advice in rcu_dereference.rst

This commit updates the rcu_access_pointer() advice, noting that its
return value should not be assigned to a local variable, and also noting
that there is little point in using rcu_access_pointer() within an RCU
read-side critical section.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 Documentation/RCU/rcu_dereference.rst | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst
index 0b418a5b243c5..81e828c8313b8 100644
--- a/Documentation/RCU/rcu_dereference.rst
+++ b/Documentation/RCU/rcu_dereference.rst
@@ -128,10 +128,16 @@ Follow these rules to keep your RCU code working properly:
 		This sort of comparison occurs frequently when scanning
 		RCU-protected circular linked lists.
 
-		Note that if checks for being within an RCU read-side
-		critical section are not required and the pointer is never
-		dereferenced, rcu_access_pointer() should be used in place
-		of rcu_dereference().
+		Note that if the pointer comparison is done outside
+		of an RCU read-side critical section, and the pointer
+		is never dereferenced, rcu_access_pointer() should be
+		used in place of rcu_dereference().  In most cases,
+		it is best to avoid accidental dereferences by testing
+		the rcu_access_pointer() return value directly, without
+		assigning it to a variable.
+
+		Within an RCU read-side critical section, there is little
+		reason to use rcu_access_pointer().
 
 	-	The comparison is against a pointer that references memory
 		that was initialized "a long time ago."  The reason
-- 
2.31.1.189.g2e36527f23