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Date: Tue, 24 Aug 2004 21:49:50 -0700
From: Andrew Morton <akpm@osdl.org>
To: Ryan Arnold <rsa@us.ibm.com>
Cc: linux-kernel@vger.kernel.org, linuxppc64-dev@lists.linuxppc.org,
       paulus@samba.org, benh@kernel.crashing.org
Subject: Re: [PATCH] interrupt driven hvc_console as vio device
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Ryan Arnold <rsa@us.ibm.com> wrote:
>
>  static int hvc_write_room(struct tty_struct *tty)
>   {
>   	struct hvc_struct *hp = tty->driver_data;
>  +	unsigned long flags;
>  +	int retval;
>   
>  -	return N_OUTBUF - hp->n_outbuf;
>  +	spin_lock_irqsave(&hp->lock, flags);
>  +	retval = N_OUTBUF - hp->n_outbuf;
>  +	spin_unlock_irqrestore(&hp->lock, flags);
>  +	return retval;
>   }
>   
>   static int hvc_chars_in_buffer(struct tty_struct *tty)
>   {
>   	struct hvc_struct *hp = tty->driver_data;
>  +	unsigned long flags;
>  +	int retval;
>   
>  -	return hp->n_outbuf;
>  +	spin_lock_irqsave(&hp->lock, flags);
>  +	retval = hp->n_outbuf;
>  +	spin_unlock_irqrestore(&hp->lock, flags);
>  +	return retval;
>   }

The new locking in these functions doesn't really do anything, apart from
adding memory barriers.  If that's what you really want, I suggest you
simply add (commented) memory barriers.