Date: Wed, 9 Aug 2006 17:29:10 -0700
From: Andrew Morton <akpm@osdl.org>
To: paulmck@us.ibm.com
Cc: stelian@popies.net, linux-kernel@vger.kernel.org, paulus@au1.ibm.com,
       anton@au1.ibm.com, open-iscsi@googlegroups.com, pradeep@us.ibm.com,
       mashirle@us.ibm.com
Subject: Re: [PATCH] memory ordering in __kfifo primitives
Message-Id: <20060809172910.614ad979.akpm@osdl.org>
In-Reply-To: <20060810001823.GA3026@us.ibm.com>
References: <20060810001823.GA3026@us.ibm.com>
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On Wed, 9 Aug 2006 17:18:23 -0700
"Paul E. McKenney" <paulmck@us.ibm.com> wrote:

> @@ -129,6 +129,8 @@ unsigned int __kfifo_put(struct kfifo *f
>  	/* then put the rest (if any) at the beginning of the buffer */
>  	memcpy(fifo->buffer, buffer + l, len - l);
>  
> +	smp_wmb();
> +
>  	fifo->in += len;
>  
>  	return len;
> @@ -161,6 +163,8 @@ unsigned int __kfifo_get(struct kfifo *f
>  	/* then get the rest (if any) from the beginning of the buffer */
>  	memcpy(buffer + l, fifo->buffer, len - l);
>  
> +	smp_mb();
> +
>  	fifo->out += len;
>  
>  	return len;

When adding barriers, please also add a little comment explaining what the
barrier is protecting us from.

Often it's fairly obvious, but sometimes it is not, and in both cases it is still
useful to communicate the programmer's intent in this way.

Thanks.
-
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