Re: [PATCH tip/core/rcu 09/12] fs: Substitute rcu_access_pointer() for rcu_dereference_raw()

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Feb 17, 2014 at 01:35:56PM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> (Trivial patch.)
> 
> If the code is looking at the RCU-protected pointer itself, but not
> dereferencing it, the rcu_dereference() functions can be downgraded to
> rcu_access_pointer().  This commit makes this downgrade in __alloc_fd(),
> which simply compares the RCU-protected pointer against NULL with no
> dereferencing.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: linux-fsdevel@vger.kernel.org

I'm beginning to wonder if this common pattern ought to have an
rcu_pointer_is_null(), which would not return the pointer, only the
boolean.

Regardless, for this patch:
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

>  fs/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index db25c2bdfe46..18f7d27855c4 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -497,7 +497,7 @@ repeat:
>  	error = fd;
>  #if 1
>  	/* Sanity check */
> -	if (rcu_dereference_raw(fdt->fd[fd]) != NULL) {
> +	if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
>  		printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
>  		rcu_assign_pointer(fdt->fd[fd], NULL);
>  	}
> -- 
> 1.8.1.5
>