RPC 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 Request Packet Structure
image::ldlm-bl-callback-request.png["LDLM_BL_CALLBACK Request Packet Structure",height=50]

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

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

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.

'ptlrpc_body'::
RPC descriptor. <<struct-ptlrpc-body>>

'ldlm_request'::
Description of the lock being requested. Which resource is the target,
what lock is current, and what lock desired. <<struct-ldlm-request>>

.LDLM_BL_CALLBACK Reply Packet Structure
image::ldlm-bl-callback-reply.png["LDLM_BL_CALLBACK Reply Packet Structure",height=50]

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

       LDLM_BL_CALLBACK:
      --reply--------
      | ptlrpc_body |
      ---------------
//////////////////////////////////////////////////////////////////////

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. <<struct-ptlrpc-body>>