Re: [PATCH 8/9] Call idr_find() without locking in ipc_lock()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, May 07, 2008 at 01:36:01PM +0200, Nadia.Derbey@bull.net wrote:
> [PATCH 08/09]
> 
> This patch makes idr_find() called locklessly in ipc_lock(), since the idr
> tree is now RCU protected.

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

> Signed-off-by: Nadia Derbey <Nadia.Derbey@bull.net>
> 
> ---
>  ipc/util.c |    9 ---------
>  1 file changed, 9 deletions(-)
> 
> Index: linux-2.6.25-mm1/ipc/util.c
> ===================================================================
> --- linux-2.6.25-mm1.orig/ipc/util.c	2008-05-06 17:15:10.000000000 +0200
> +++ linux-2.6.25-mm1/ipc/util.c	2008-05-07 09:56:20.000000000 +0200
> @@ -688,10 +688,6 @@ void ipc64_perm_to_ipc_perm (struct ipc6
>   * Look for an id in the ipc ids idr and lock the associated ipc object.
>   *
>   * The ipc object is locked on exit.
> - *
> - * This is the routine that should be called when the rw_mutex is not already
> - * held, i.e. idr tree not protected: it protects the idr tree in read mode
> - * during the idr_find().
>   */
> 
>  struct kern_ipc_perm *ipc_lock(struct ipc_ids *ids, int id)
> @@ -699,18 +695,13 @@ struct kern_ipc_perm *ipc_lock(struct ip
>  	struct kern_ipc_perm *out;
>  	int lid = ipcid_to_idx(id);
> 
> -	down_read(&ids->rw_mutex);
> -
>  	rcu_read_lock();
>  	out = idr_find(&ids->ipcs_idr, lid);
>  	if (out == NULL) {
>  		rcu_read_unlock();
> -		up_read(&ids->rw_mutex);
>  		return ERR_PTR(-EINVAL);
>  	}
> 
> -	up_read(&ids->rw_mutex);
> -
>  	spin_lock(&out->lock);
>  	
>  	/* ipc_rmid() may have already freed the ID while ipc_lock
> 
> --