LU-3443 mdc: document mdc_rpc_lock

As this lock can be a bottleneck, clarifying why it is needed may be
helpful to those working on client performance.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Change-Id: Ib617a99a51b65b1141e52a6f31d4431828203650
Reviewed-on: http://review.whamcloud.com/6593
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>

diff --git a/lustre/include/lustre_mdc.h b/lustre/include/lustre_mdc.h
index 6cb5f01..bc04678 100644 (file)
--- a/lustre/include/lustre_mdc.h
+++ b/lustre/include/lustre_mdc.h
@@ -69,9 +69,27 @@ struct obd_export;
 struct ptlrpc_request;
 struct obd_device;
 
+/**
+ * Serializes in-flight MDT-modifying RPC requests to preserve idempotency.
+ *
+ * This mutex is used to implement execute-once semantics on the MDT.
+ * The MDT stores the last transaction ID and result for every client in
+ * its last_rcvd file. If the client doesn't get a reply, it can safely
+ * resend the request and the MDT will reconstruct the reply being aware
+ * that the request has already been executed. Without this lock,
+ * execution status of concurrent in-flight requests would be
+ * overwritten.
+ *
+ * This design limits the extent to which we can keep a full pipeline of
+ * in-flight requests from a single client.  This limitation could be
+ * overcome by allowing multiple slots per client in the last_rcvd file.
+ */
 struct mdc_rpc_lock {
+       /** Lock protecting in-flight RPC concurrency. */
        struct mutex            rpcl_mutex;
+       /** Intent associated with currently executing request. */
        struct lookup_intent    *rpcl_it;
+       /** Used for MDS/RPC load testing purposes. */
        int                     rpcl_fakes;
 };