dlm: move lkb xarray lookup out of lock

This patch moves the xarray lookup functionality for the lkb out of the
ls_lkbxa_lock read lock handling. We can do that as the xarray should be
possible to access lockless in case of reader like xa_load(). We confirm
under ls_lkbxa_lock that the lkb is still part of the data structure and
take a reference when its still part of ls_lkbxa to avoid being freed
after doing the lookup. To do a check if the lkb is still part of the
ls_lkbxa data structure we use a kref_read() as the last put will remove
it from the ls_lkbxa data structure and any reference taken means it is
still part of ls_lkbxa.

A similar approach was done with the DLM rsb rhashtable just with a flag
instead of the refcounter because the refcounter has a slightly
different meaning.

Signed-off-by: Alexander Aring <[e-mail]>
Signed-off-by: David Teigland <[e-mail]>

fs/dlm/dlm_internal.h

···
 		void			*lkb_astparam;	/* caller's ast arg */
 		struct dlm_user_args	*lkb_ua;
 	};
+	struct rcu_head		rcu;
 };
 
 /*

fs/dlm/lock.c

···
 {
 	struct dlm_lkb *lkb;
 
-	read_lock_bh(&ls->ls_lkbxa_lock);
+	rcu_read_lock();
 	lkb = xa_load(&ls->ls_lkbxa, lkid);
-	if (lkb)
-		kref_get(&lkb->lkb_ref);
-	read_unlock_bh(&ls->ls_lkbxa_lock);
+	if (lkb) {
+		/* check if lkb is still part of lkbxa under lkbxa_lock as
+		 * the lkb_ref is tight to the lkbxa data structure, see
+		 * __put_lkb().
+		 */
+		read_lock_bh(&ls->ls_lkbxa_lock);
+		if (kref_read(&lkb->lkb_ref))
+			kref_get(&lkb->lkb_ref);
+		else
+			lkb = NULL;
+		read_unlock_bh(&ls->ls_lkbxa_lock);
+	}
+	rcu_read_unlock();
 
 	*lkb_ret = lkb;
 	return lkb ? 0 : -ENOENT;

fs/dlm/memory.c

···
 	return kmem_cache_zalloc(lkb_cache, GFP_ATOMIC);
 }
 
-void dlm_free_lkb(struct dlm_lkb *lkb)
+static void __free_lkb_rcu(struct rcu_head *rcu)
 {
+	struct dlm_lkb *lkb = container_of(rcu, struct dlm_lkb, rcu);
+
 	if (test_bit(DLM_DFL_USER_BIT, &lkb->lkb_dflags)) {
 		struct dlm_user_args *ua;
 		ua = lkb->lkb_ua;
···
 	}
 
 	kmem_cache_free(lkb_cache, lkb);
+}
+
+void dlm_free_lkb(struct dlm_lkb *lkb)
+{
+	call_rcu(&lkb->rcu, __free_lkb_rcu);
 }
 
 struct dlm_mhandle *dlm_allocate_mhandle(void)