Re: [PATCH RFC] rcu: add sparse check to rcu_assign_pointer

[Paul E. McKenney]

On Wed, Dec 12, 2018 at 02:37:10PM -0800, Joel Fernandes (Google) wrote:
> rcu_assign_pointer currently doesn't do any sparse checking on a pointer
> assigned. So its possible that a pointer that is not __rcu annotated is
> assigned with rcu_assign_pointer without sparse complainting.
> rcu_dereference already does such checking so lets also make
> rcu_assign_pointer to do the same. The extra error could be helpful in
> cases where an RCU pointer is assigned with rcu_assign_pointer but not
> annotated with __rcu.
> 
> This doesn't generate any code in the normal case because __CHECKER__ is
> defined only in the context of sparse.
> 
> Also we rename rcu_dereference_sparse to rcu_check_parse since the
> checking now happens not only during derereferencing but also during
> assignment.
> 
> Test: Introduced an rcu_assign_pointer in code and checked the output of
> sparse with and without this change. The change correctly causes sparse
> to throw an error.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

Applied with some commit-log fixes, thank you!

							Thanx, Paul

> ---
>  include/linux/rcupdate.h | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 4db8bcacc51a..9e6f10da7f26 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -309,16 +309,16 @@ static inline void rcu_preempt_sleep_check(void) { }
>   */
> 
>  #ifdef __CHECKER__
> -#define rcu_dereference_sparse(p, space) \
> +#define rcu_check_sparse(p, space) \
>  	((void)(((typeof(*p) space *)p) == p))
>  #else /* #ifdef __CHECKER__ */
> -#define rcu_dereference_sparse(p, space)
> +#define rcu_check_sparse(p, space)
>  #endif /* #else #ifdef __CHECKER__ */
> 
>  #define __rcu_access_pointer(p, space) \
>  ({ \
>  	typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
> -	rcu_dereference_sparse(p, space); \
> +	rcu_check_sparse(p, space); \
>  	((typeof(*p) __force __kernel *)(_________p1)); \
>  })
>  #define __rcu_dereference_check(p, c, space) \
> @@ -326,13 +326,13 @@ static inline void rcu_preempt_sleep_check(void) { }
>  	/* Dependency order vs. p above. */ \
>  	typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
>  	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
> -	rcu_dereference_sparse(p, space); \
> +	rcu_check_sparse(p, space); \
>  	((typeof(*p) __force __kernel *)(________p1)); \
>  })
>  #define __rcu_dereference_protected(p, c, space) \
>  ({ \
>  	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
> -	rcu_dereference_sparse(p, space); \
> +	rcu_check_sparse(p, space); \
>  	((typeof(*p) __force __kernel *)(p)); \
>  })
>  #define rcu_dereference_raw(p) \
> @@ -382,6 +382,7 @@ static inline void rcu_preempt_sleep_check(void) { }
>  #define rcu_assign_pointer(p, v)					      \
>  ({									      \
>  	uintptr_t _r_a_p__v = (uintptr_t)(v);				      \
> +	rcu_check_sparse(p, __rcu);				      \
>  									      \
>  	if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL)	      \
>  		WRITE_ONCE((p), (typeof(p))(_r_a_p__v));		      \
> @@ -785,7 +786,7 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
>   */
>  #define RCU_INIT_POINTER(p, v) \
>  	do { \
> -		rcu_dereference_sparse(p, __rcu); \
> +		rcu_check_sparse(p, __rcu); \
>  		WRITE_ONCE(p, RCU_INITIALIZER(v)); \
>  	} while (0)
> 
> -- 
> 2.20.0.rc1.387.gf8505762e3-goog
>