linux-kernel - Re: [PATCH] rculist: Add brackets around cond argument in __list_check

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Date:   Sat, 18 Jan 2020 21:14:25 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Amol Grover <frextrite@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>
Subject: Re: [PATCH] rculist: Add brackets around cond argument in
 __list_check_rcu macro

On Sat, Jan 18, 2020 at 10:24:18PM +0530, Amol Grover wrote:
> 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@...il.com>

Good catch!

Acked-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

 - Joel


> ---
>  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
>