linux-kernel - Re: [PATCH tip/core/rcu 12/15] rcu: Remove redundant memory barrier from __call

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Message-ID: <20120831183035.GJ4259@jtriplet-mobl1>
Date:	Fri, 31 Aug 2012 11:30:35 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 12/15] rcu: Remove redundant memory barrier
 from __call_rcu()

On Thu, Aug 30, 2012 at 11:56:25AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@...aro.org>
> 
> The first memory barrier in __call_rcu() is supposed to order any
> updates done beforehand by the caller against the actual queuing
> of the callback.  However, the second memory barrier (which is intended
> to order incrementing the queue lengths before queuing the callback)
> is also between the caller's updates and the queuing of the callback.
> The second memory barrier can therefore serve both purposes.
> 
> This commit therefore removes the first memory barrier.

I don't see any such second memory barrier in __call_rcu(), at least not
in current master.  Right after this smp_mb(), __call_rcu() enqueues the
callback and increments the queue length.

Did you add a second memory barrier in some other patch that hasn't made
it upstream yet?  If so, could you note that patch dependency explicitly
in the commit message?

- Josh Triplett

> Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  kernel/rcutree.c |    2 --
>  1 files changed, 0 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index e58097b..5b6709b 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1923,8 +1923,6 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
>  	head->func = func;
>  	head->next = NULL;
>  
> -	smp_mb(); /* Ensure RCU update seen before callback registry. */
> -
>  	/*
>  	 * Opportunistically note grace-period endings and beginnings.
>  	 * Note that we might see a beginning right after we see an
> -- 
> 1.7.8
> 
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