linux-kernel - Re: [PATCH] fs: avoid unecessary smp_wmb() in dup

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Message-ID: <20110508142623.GO2641@linux.vnet.ibm.com>
Date:	Sun, 8 May 2011 07:26:23 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <matthew@....cx>, linux-fsdevel@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Changli Gao <xiaosuo@...il.com>
Subject: Re: [PATCH] fs: avoid unecessary smp_wmb() in dup_fd()

On Sat, May 07, 2011 at 08:30:01AM +0200, Eric Dumazet wrote:
> While copying old_fds array to new_fds, we dont need
> rcu_assign_pointer() and its smp_wmb(), since we are the only thread
> populating new_fds at this point.
> 
> A bit later, rcu_assign_pointer(newf->fdt, new_fdt) makes sure
> our writes are committed to memory before new_fds content is visible to
> other threads.
> 
> This removes following sparse warning :
> 
> fs/file.c:371:3: warning: incorrect type in assignment (different
> address spaces)
> fs/file.c:371:3:    expected struct file *<noident>
> fs/file.c:371:3:    got struct file [noderef] <asn:4>*<noident>

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

> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> Cc: Matthew Wilcox <matthew@....cx>
> Cc: linux-fsdevel@...r.kernel.org
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Changli Gao <xiaosuo@...il.com>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  fs/file.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index 4c6992d..8320a8e 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -368,7 +368,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
>  			 */
>  			FD_CLR(open_files - i, new_fdt->open_fds);
>  		}
> -		rcu_assign_pointer(*new_fds++, f);
> +		*new_fds++ = f;
>  	}
>  	spin_unlock(&oldf->file_lock);
> 
> 
> 
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