ipc: call idr_find() without locking in ipc_lock()

Call idr_find() locklessly from ipc_lock(), since the idr tree is now RCU
protected.
Signed-off-by: NNadia Derbey <Nadia.Derbey@bull.net>
Acked-by: N"Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Jim Houston <jim.houston@comcast.net>
Cc: Pierre Peiffer <peifferp@gmail.com>
Acked-by: NRik van Riel <riel@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

ipc/util.c

@@ -688,10 +688,6 @@ void ipc64_perm_to_ipc_perm (struct ipc64_perm *in, struct ipc_perm *out)
  * 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 ipc_ids *ids, int id)
 	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