staging: lustre: mdc: document mdc_rpc_lock

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;

};