Command 104: LDLM_BL_CALLBACK
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[[ldlm-bl-callback-rpc]]

An RPC that assists with getting a lock back from an entity that has
it.

.LDLM_BL_CALLBACK Generic Packet Structure
image::ldlm-bl-callback-generic.png["LDLM_BL_CALLBACK Generic Packet Structure",height=100]

//////////////////////////////////////////////////////////////////////
The ldlm-bl-callback-generic.png diagram resembles this text
art:

       LDLM_BL_CALLBACK:
      --request---------------------
      | ptlrpc_body | ldlm_request |
      ------------------------------
      --reply--------
      | ptlrpc_body |
      ---------------
//////////////////////////////////////////////////////////////////////

The request RPC resembles the simplest LDLM_ENQUEUE RPC, but only
identifies the relevant resource that the destination entity already
had a lock on. It notifies the recipient that the lock should be
returned once and dirty write data has been flushed.  The Reply is
just an acknowledgment of receipt, and doesn't carry any further
information about the lock or the resource.

'ptlrpc_body'::
RPC descriptor.

'ldlm_request'::
Description of the lock being requested. Which resource is the target,
what lock is current, and what lock desired.