2020-01-18 16:57:04

by Amol Grover

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

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 <[email protected]>
---
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

2020-01-19 02:15:31

by Joel Fernandes

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 <[email protected]>

Good catch!

Acked-by: Joel Fernandes (Google) <[email protected]>

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
>

2020-01-21 00:03:10

by Paul E. McKenney

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

On Sat, Jan 18, 2020 at 09:14:25PM -0500, Joel Fernandes wrote:
> 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 <[email protected]>
>
> Good catch!
>
> Acked-by: Joel Fernandes (Google) <[email protected]>

Queued for v5.7, thank you both!

Thanx, Paul

> 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
> >