IB/cm: Improve local id allocation

The IB CM uses an idr for local id allocations, with a running counter
as start_id.  This fails to generate distinct ids if

1. An id is constantly created and destroyed
2. A chunk of ids just beyond the current next_id value is occupied

This in turn leads to an increased chance of connection request being
mis-detected as a duplicate, sometimes for several retries, until
next_id gets past the block of allocated ids. This has been observed
in practice.

As a fix, remember the last id allocated and start immediately above it.
This also fixes a problem with the old code, where next_id might
overflow and become negative.

Signed-off-by: Michael S. Tsirkin <mst@dev.mellanox.co.il>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/core/cm.c

@@ -306,7 +306,9 @@ static int cm_alloc_id(struct cm_id_private *cm_id_priv)
 	do {
 		spin_lock_irqsave(&cm.lock, flags);
 		ret = idr_get_new_above(&cm.local_id_table, cm_id_priv,
-					next_id++, &id);
+					next_id, &id);
+		if (!ret)
+			next_id = ((unsigned) id + 1) & MAX_ID_MASK;
 		spin_unlock_irqrestore(&cm.lock, flags);
 	} while( (ret == -EAGAIN) && idr_pre_get(&cm.local_id_table, GFP_KERNEL) );