linux-kernel - Re: [PATCH 1/2] srcu: API for barrier after srcu read unlock

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Message-ID: <20131104205524.GY3947@linux.vnet.ibm.com>
Date:	Mon, 4 Nov 2013 12:55:24 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Lai Jiangshan <laijs@...fujitsu.com>
Subject: Re: [PATCH 1/2] srcu: API for barrier after srcu read unlock

On Mon, Nov 04, 2013 at 10:36:17PM +0200, Michael S. Tsirkin wrote:
> srcu read lock/unlock include a full memory barrier
> but that's an implementation detail.
> Add an API for make memory fencing explicit for
> users that need this barrier, to make sure we
> can change it as needed without breaking all users.
> 
> Acked-by: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> ---
>  include/linux/srcu.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index c114614..9b058ee 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -237,4 +237,18 @@ static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
>  	__srcu_read_unlock(sp, idx);
>  }
> 
> +/**
> + * smp_mb__after_srcu_read_unlock - ensure full ordering after srcu_read_unlock
> + *
> + * Converts the preceding srcu_read_unlock into a two-way memory barrier.
> + *
> + * Call this after srcu_read_unlock, to guarantee that all memory operations
> + * that occur after smp_mb__after_srcu_read_unlock will appear to happen after
> + * the preceding srcu_read_unlock.
> + */
> +static inline void smp_mb__after_srcu_read_unlock(void)
> +{
> +	/* __srcu_read_unlock has smp_mb() internally so nothing to do here. */
> +}
> +
>  #endif
> -- 
> MST
> 

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