linux-kernel - Re: [PATCH tip/core/rcu 2/3] srcu: Force full grace-period ordering

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Date:   Sun, 15 Jan 2017 08:11:23 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
        fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
        will.deacon@....com, boqun.feng@...ux.vnet.ibm.com,
        linuxppc-dev@...ts.ozlabs.org,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH tip/core/rcu 2/3] srcu: Force full grace-period ordering


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 357b32aaea48..5fdfe874229e 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -1175,11 +1175,11 @@ do { \
>   * if the UNLOCK and LOCK are executed by the same CPU or if the
>   * UNLOCK and LOCK operate on the same lock variable.
>   */
> -#ifdef CONFIG_PPC
> +#ifdef CONFIG_ARCH_WEAK_RELACQ
>  #define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
> -#else /* #ifdef CONFIG_PPC */
> +#else /* #ifdef CONFIG_ARCH_WEAK_RELACQ */
>  #define smp_mb__after_unlock_lock()	do { } while (0)
> -#endif /* #else #ifdef CONFIG_PPC */
> +#endif /* #else #ifdef CONFIG_ARCH_WEAK_RELACQ */
>  
>  

So at the risk of sounding totally pedantic, why not structure it like the 
existing smp_mb__before/after*() primitives in barrier.h?

That allows asm-generic/barrier.h to pick up the definition - for example in the 
case of smp_acquire__after_ctrl_dep() we do:

 #ifndef smp_acquire__after_ctrl_dep
 #define smp_acquire__after_ctrl_dep()           smp_rmb()
 #endif

Which allows Tile to relax it:

  arch/tile/include/asm/barrier.h:#define smp_acquire__after_ctrl_dep()   barrier()

I.e. I'd move the API definition out of rcupdate.h and into barrier.h - even 
though tree-RCU is the only user of this barrier type.

Thanks,

	Ingo