Re: [PATCH v3 1/3] rcu: Introduce rcu_swap_protected()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Aug 28, 2017 at 01:46:13PM -0700, Bart Van Assche wrote:
> A common pattern in RCU code is to assign a new value to an RCU
> pointer after having read and stored the old value. Introduce a
> macro for this pattern.
> 
> Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Hannes Reinecke <hare@suse.de>
> Cc: Johannes Thumshirn <jthumshirn@suse.de>
> Cc: Shane M Seymour <shane.seymour@hpe.com>
> ---
>  include/linux/rcupdate.h | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index f816fc72b51e..555815ce2e57 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -561,6 +561,26 @@ static inline void rcu_preempt_sleep_check(void) { }
>   */
>  #define rcu_pointer_handoff(p) (p)
> 
> +/**
> + * rcu_swap_protected() - swap an RCU and a regular pointer
> + * @rcu_ptr: RCU pointer
> + * @ptr: regular pointer
> + * @c: the conditions under which the dereference will take place
> + *
> + * Perform swap(@rcu_ptr, @ptr) where @rcu_ptr is an RCU-annotated pointer and
> + * @c is the argument that is passed to the rcu_dereference_protected() call
> + * used to read that pointer.
> + */
> +#define rcu_swap_protected(rcu_ptr, ptr, c) do {			\
> +	typeof(ptr) __tmp;						\
> +									\
> +	BUILD_BUG_ON(!__same_type(typeof(rcu_ptr), typeof(*(ptr)) __rcu *)); \
> +	BUILD_BUG_ON(!__same_type(typeof(ptr), typeof(*(rcu_ptr)) *));	\

Hmmm...

What kinds of bugs have these two BUILD_BUG_ON() instances have caught
that would not be caught by the assignments below?

> +	__tmp = rcu_dereference_protected((rcu_ptr), (c));		\
> +	rcu_assign_pointer((rcu_ptr), (ptr));				\
> +	(ptr) = __tmp;							\
> +} while (0)
> +

Could you please put this after rcu_assign_pointer() and before
rcu_access_pointer()?  That way the things that assign to RCU-protected
pointers are together.

							Thanx, Paul

>  /**
>   * rcu_read_lock() - mark the beginning of an RCU read-side critical section
>   *
> -- 
> 2.14.1
>