xprtrdma: Refresh the documenting comment in frwr_ops.c

Things have changed since this comment was written. In particular,
the reworking of connection closing, on-demand creation of MRs, and
the removal of fr_state all mean that deferring MR recovery to
frwr_map is no longer needed. The description is obsolete.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/frwr_ops.c

@@ -7,67 +7,37 @@
 /* Lightweight memory registration using Fast Registration Work
  * Requests (FRWR).
  *
- * FRWR features ordered asynchronous registration and deregistration
- * of arbitrarily sized memory regions. This is the fastest and safest
+ * FRWR features ordered asynchronous registration and invalidation
+ * of arbitrarily-sized memory regions. This is the fastest and safest
  * but most complex memory registration mode.
  */
 
 /* Normal operation
  *
- * A Memory Region is prepared for RDMA READ or WRITE using a FAST_REG
+ * A Memory Region is prepared for RDMA Read or Write using a FAST_REG
  * Work Request (frwr_map). When the RDMA operation is finished, this
  * Memory Region is invalidated using a LOCAL_INV Work Request
- * (frwr_unmap_sync).
+ * (frwr_unmap_async and frwr_unmap_sync).
  *
- * Typically these Work Requests are not signaled, and neither are RDMA
- * SEND Work Requests (with the exception of signaling occasionally to
- * prevent provider work queue overflows). This greatly reduces HCA
+ * Typically FAST_REG Work Requests are not signaled, and neither are
+ * RDMA Send Work Requests (with the exception of signaling occasionally
+ * to prevent provider work queue overflows). This greatly reduces HCA
  * interrupt workload.
- *
- * As an optimization, frwr_unmap marks MRs INVALID before the
- * LOCAL_INV WR is posted. If posting succeeds, the MR is placed on
- * rb_mrs immediately so that no work (like managing a linked list
- * under a spinlock) is needed in the completion upcall.
- *
- * But this means that frwr_map() can occasionally encounter an MR
- * that is INVALID but the LOCAL_INV WR has not completed. Work Queue
- * ordering prevents a subsequent FAST_REG WR from executing against
- * that MR while it is still being invalidated.
  */
 
 /* Transport recovery
  *
- * ->op_map and the transport connect worker cannot run at the same
- * time, but ->op_unmap can fire while the transport connect worker
- * is running. Thus MR recovery is handled in ->op_map, to guarantee
- * that recovered MRs are owned by a sending RPC, and not one where
- * ->op_unmap could fire at the same time transport reconnect is
- * being done.
- *
- * When the underlying transport disconnects, MRs are left in one of
- * four states:
- *
- * INVALID:	The MR was not in use before the QP entered ERROR state.
- *
- * VALID:	The MR was registered before the QP entered ERROR state.
- *
- * FLUSHED_FR:	The MR was being registered when the QP entered ERROR
- *		state, and the pending WR was flushed.
- *
- * FLUSHED_LI:	The MR was being invalidated when the QP entered ERROR
- *		state, and the pending WR was flushed.
- *
- * When frwr_map encounters FLUSHED and VALID MRs, they are recovered
- * with ib_dereg_mr and then are re-initialized. Because MR recovery
- * allocates fresh resources, it is deferred to a workqueue, and the
- * recovered MRs are placed back on the rb_mrs list when recovery is
- * complete. frwr_map allocates another MR for the current RPC while
- * the broken MR is reset.
- *
- * To ensure that frwr_map doesn't encounter an MR that is marked
- * INVALID but that is about to be flushed due to a previous transport
- * disconnect, the transport connect worker attempts to drain all
- * pending send queue WRs before the transport is reconnected.
+ * frwr_map and frwr_unmap_* cannot run at the same time the transport
+ * connect worker is running. The connect worker holds the transport
+ * send lock, just as ->send_request does. This prevents frwr_map and
+ * the connect worker from running concurrently. When a connection is
+ * closed, the Receive completion queue is drained before the allowing
+ * the connect worker to get control. This prevents frwr_unmap and the
+ * connect worker from running concurrently.
+ *
+ * When the underlying transport disconnects, MRs that are in flight
+ * are flushed and are likely unusable. Thus all flushed MRs are
+ * destroyed. New MRs are created on demand.
  */
 
 #include <linux/sunrpc/rpc_rdma.h>