linux-kernel - Re: [PATCH 2/3] relay: Fix race condition which occurs when reading across CPUs.

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Message-ID: <4851A9FA.1000104@gmail.com>
Date:	Fri, 13 Jun 2008 00:58:01 +0200 (MEST)
From:	Andrea Righi <righi.andrea@...il.com>
To:	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Cc:	tzanussi@...il.com, penberg@...helsinki.fi,
	akpm@...ux-foundation.org, compudj@...stal.dyndns.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] relay: Fix race condition which occurs when reading
 across CPUs.

Eduard - Gabriel Munteanu wrote:
> @@ -1028,15 +1032,19 @@ static ssize_t relay_file_read_subbufs(struct file *filp, loff_t *ppos,
>  			break;
>  
>  		avail = min(desc->count, avail);
> +		/* subbuf_actor may sleep, so release the spinlock for now */
> +		spin_unlock_irqrestore(&buf->rw_lock, flags);
>  		ret = subbuf_actor(read_start, buf, avail, desc, actor);
>  		if (desc->error < 0)
>  			break;

If you just break here buf->rw_lock will be unlocked twice. Maybe a
better way is:
		if (desc->error < 0)
			goto out;
		...
  	} while (desc->count && ret);
	spin_unlock_irqrestore(&buf->rw_lock, flags);
out:
  	mutex_unlock(&filp->f_path.dentry->d_inode->i_mutex);

  	return desc->written;

> +		spin_lock_irqsave(&buf->rw_lock, flags);
>  
>  		if (ret) {
>  			relay_file_read_consume(buf, read_start, ret);
>  			*ppos = relay_file_read_end_pos(buf, read_start, ret);
>  		}
>  	} while (desc->count && ret);
> +	spin_unlock_irqrestore(&buf->rw_lock, flags);
>  	mutex_unlock(&filp->f_path.dentry->d_inode->i_mutex);
>  
>  	return desc->written;

-Andrea
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