Re: [PATCH] rcu: Prevent evaluation of rcu_assign_pointer()

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

On Fri, May 24, 2019 at 12:36:37PM +0200, Andrea Parri wrote:
> Quoting Paul [1]:
> 
>  "Given that a quick (and perhaps error-prone) search of the uses
>   of rcu_assign_pointer() in v5.1 didn't find a single use of the
>   return value, let's please instead change the documentation and
>   implementation to eliminate the return value."
> 
> [1] https://lkml.kernel.org/r/20190523135013.GL28207@linux.ibm.com

Thank you!  A few comments below.

							Thanx, Paul

> Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
> Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> Cc: rcu@vger.kernel.org
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Sasha Levin <sashal@kernel.org>
> ---
> Matthew, Sasha:
> 
> The patch is based on -rcu/dev; I took the liberty of applying the
> same change to your #defines in:
> 
>  tools/testing/radix-tree/linux/rcupdate.h
>  tools/include/linux/rcu.h
> 
> but I admit that I'm not familiar with their uses: please shout if
> you have any objections with it.
> ---
>  Documentation/RCU/whatisRCU.txt           |  8 ++++----
>  include/linux/rcupdate.h                  |  5 ++---
>  tools/include/linux/rcu.h                 | 11 +++++++++--
>  tools/testing/radix-tree/linux/rcupdate.h |  5 ++++-
>  4 files changed, 19 insertions(+), 10 deletions(-)
> 
> diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
> index 981651a8b65d2..f99a87b9a88fa 100644
> --- a/Documentation/RCU/whatisRCU.txt
> +++ b/Documentation/RCU/whatisRCU.txt
> @@ -212,7 +212,7 @@ synchronize_rcu()
>  
>  rcu_assign_pointer()
>  
> -	typeof(p) rcu_assign_pointer(p, typeof(p) v);
> +	rcu_assign_pointer(p, typeof(p) v);

Please add the "void", similar to synchronize_rcu() above.

>  	Yes, rcu_assign_pointer() -is- implemented as a macro, though it
>  	would be cool to be able to declare a function in this manner.
> @@ -220,9 +220,9 @@ rcu_assign_pointer()
>  
>  	The updater uses this function to assign a new value to an
>  	RCU-protected pointer, in order to safely communicate the change
> -	in value from the updater to the reader.  This function returns
> -	the new value, and also executes any memory-barrier instructions
> -	required for a given CPU architecture.
> +	in value from the updater to the reader.  This macro does not
> +	evaluate to an rvalue, but it does execute any memory-barrier
> +	instructions required for a given CPU architecture.
>  
>  	Perhaps just as important, it serves to document (1) which
>  	pointers are protected by RCU and (2) the point at which a
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 915460ec08722..a5f61a08e65fc 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -367,7 +367,7 @@ static inline void rcu_preempt_sleep_check(void) { }
>   * other macros that it invokes.
>   */
>  #define rcu_assign_pointer(p, v)					      \
> -({									      \
> +do {									      \
>  	uintptr_t _r_a_p__v = (uintptr_t)(v);				      \
>  	rcu_check_sparse(p, __rcu);				      \
>  									      \
> @@ -375,8 +375,7 @@ static inline void rcu_preempt_sleep_check(void) { }
>  		WRITE_ONCE((p), (typeof(p))(_r_a_p__v));		      \
>  	else								      \
>  		smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
> -	_r_a_p__v;							      \
> -})
> +} while (0)
>  
>  /**
>   * rcu_swap_protected() - swap an RCU and a regular pointer
> diff --git a/tools/include/linux/rcu.h b/tools/include/linux/rcu.h
> index 7d02527e5bcea..01a435ee48cd6 100644
> --- a/tools/include/linux/rcu.h
> +++ b/tools/include/linux/rcu.h
> @@ -19,7 +19,14 @@ static inline bool rcu_is_watching(void)
>  	return false;
>  }
>  
> -#define rcu_assign_pointer(p, v) ((p) = (v))
> -#define RCU_INIT_POINTER(p, v) p=(v)
> +#define rcu_assign_pointer(p, v)				\
> +do {								\
> +	(p) = (v);						\
> +} while (0)
> +
> +#define RCU_INIT_POINTER(p, v)					\
> +do {								\
> +	(p) = (v);						\
> +} while (0)

These two each fit nicely on one line:

	#define rcu_assign_pointer(p, v) do { (p) = (v); } while (0)
	#define RCU_INIT_POINTER(p, v)   do { (p) = (v); } while (0)

>  
>  #endif
> diff --git a/tools/testing/radix-tree/linux/rcupdate.h b/tools/testing/radix-tree/linux/rcupdate.h
> index fd280b070fdb1..48212f3a758e6 100644
> --- a/tools/testing/radix-tree/linux/rcupdate.h
> +++ b/tools/testing/radix-tree/linux/rcupdate.h
> @@ -7,6 +7,9 @@
>  #define rcu_dereference_raw(p) rcu_dereference(p)
>  #define rcu_dereference_protected(p, cond) rcu_dereference(p)
>  #define rcu_dereference_check(p, cond) rcu_dereference(p)
> -#define RCU_INIT_POINTER(p, v)	(p) = (v)
> +#define RCU_INIT_POINTER(p, v)					\
> +do {								\
> +	(p) = (v);						\
> +} while (0)

As does this one.

>  #endif
> -- 
> 2.7.4
>