RPC 104: LDLM_GL_CALLBACK
~~~~~~~~~~~~~~~~~~~~~~~~~
[[ldlm-gl-callback-rpc]]

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

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

//////////////////////////////////////////////////////////////////////
The ldlm-gl-callback-generic.png diagram resemgles this text
art:

       LDLM_GL_CALLBACK:
      --request---------------------
      | ptlrpc_body | ldlm_request |
      ------------------------------
      --reply------------------
      | ptlrpc_body | ost_lvb |
      -------------------------
//////////////////////////////////////////////////////////////////////

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 asks the recipient to flush its dirty write cache,
and notify the requester of size and time attributes once that is
done.  The reply updates the attributes on the requester.

'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.

'ost_lvb'::
Attribute data associated with a resource on an OST.