linux-kernel - Re: [PATCH tip/core/rcu 07/12] rcu: No ordering for rcu_assign

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Message-ID: <20160615220339.GZ30921@twins.programming.kicks-ass.net>
Date:	Thu, 16 Jun 2016 00:03:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 07/12] rcu: No ordering for
 rcu_assign_pointer() of NULL

On Wed, Jun 15, 2016 at 02:46:08PM -0700, Paul E. McKenney wrote:
> This commit does a compile-time check for rcu_assign_pointer() of NULL,
> and uses WRITE_ONCE() rather than smp_store_release() in that case.
> 
> Reported-by: Christoph Hellwig <hch@...radead.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  include/linux/rcupdate.h | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index c61b6b9506e7..9be61e47badc 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -650,7 +650,16 @@ static inline void rcu_preempt_sleep_check(void)
>   * please be careful when making changes to rcu_assign_pointer() and the
>   * other macros that it invokes.
>   */
> -#define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v))
> +#define rcu_assign_pointer(p, v) \
> +({ \
> +	uintptr_t _r_a_p__v = (uintptr_t)(v); \
> +	\
> +	if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL) \
> +		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; \
> +})

Can we pretty please right align the '\'s ?

Also, didn't we used to do this and then reverted it again for some
obscure reason?