[PATCH] rculist: Add brackets around cond argument in __list_check_rcu macro

From: Amol Grover <frextrite@gmail.com>
To: "Paul E. McKenney" <paulmck@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Joel Fernandes <joel@joelfernandes.org>
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-kernel-mentees@lists.linuxfoundation.org,
	Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>,
	Amol Grover <frextrite@gmail.com>
Subject: [PATCH] rculist: Add brackets around cond argument in __list_check_rcu macro
Date: Sat, 18 Jan 2020 22:24:18 +0530	[thread overview]
Message-ID: <20200118165417.12325-1-frextrite@gmail.com> (raw)

Passing a complex lockdep condition to __list_check_rcu results
in false positive lockdep splat due to incorrect expression
evaluation.

For example, a lockdep check condition `cond1 || cond2` is
evaluated as `!cond1 || cond2 && !rcu_read_lock_any_held()`
which, according to operator precedence, evaluates to
`!cond1 || (cond2 && !rcu_read_lock_any_held())`.
This would result in a lockdep splat when cond1 is false
and cond2 is true which is logically incorrect.

Signed-off-by: Amol Grover <frextrite@gmail.com>
---
 include/linux/rculist.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 4158b7212936..dce491f0b354 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -50,9 +50,9 @@ static inline void INIT_LIST_HEAD_RCU(struct list_head *list)
 #define __list_check_rcu(dummy, cond, extra...)				\
 	({								\
 	check_arg_count_one(extra);					\
-	RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(),		\
+	RCU_LOCKDEP_WARN(!(cond) && !rcu_read_lock_any_held(),		\
 			 "RCU-list traversed in non-reader section!");	\
-	 })
+	})
 #else
 #define __list_check_rcu(dummy, cond, extra...)				\
 	({ check_arg_count_one(extra); })
-- 
2.24.1

