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srpt/session-management.txt: Document how sessions are managed by the ib_srpt driver

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git-svn-id: http://svn.code.sf.net/p/scst/svn/trunk@5358 d57e44dd-8a1f-0410-8b47-8ef2f437770f
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Bart Van Assche
2014-03-20 15:26:48 +00:00
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ib_srpt and session management
==============================
The following actions related to SRP sessions can all occur concurrently:
* IB communication manager (CM) invokes srpt_cm_handler().
* HCA driver invokes the queue pair (QP) completion handler srpt_completion().
* HCA driver invokes the QP async event handler srpt_qp_event().
* HCA transfers data between initiator and target via RDMA.
* srpt_compl_thread() polls the QP.
* SCST core invokes one of the callback functions defined in srpt_template().
The actions that occur over the lifetime of a session are as follows:
- A REQ message is received from the initiator.
- srpt_cm_req_recv() is invoked and allocates a queue pair and creates a
completion thread.
- If the connection request is not accepted, a REJ message is sent and
srpt_close_ch() is invoked. The srpt_close_ch() call causes the completion
thread to stop and the allocated resources to be freed asynchronously.
- If the connection request is accepted a REP message is sent to the initiator.
- Once an RTU message is received from the initiator, srpt_cm_rtu_recv() is
invoked. That function changes the queue pair state into RTS, the channel
state into CH_LIVE and wakes up the completion thread.
- RDMA communication starts and continues until either a DREQ message is
received or sent. The function ib_send_cm_dreq() can get invoked
either because a target port is disabled or from inside the
srpt_close_session() function.
- After a DREQ has been sent either a DREP will be received
(srpt_cm_drep_recv()) or the TimeWait state will be reached and will be left
(srpt_cm_timewait_exit()).
- srpt_cm_dre[pq]_recv() and srpt_cm_timewait_exit() all invoke
srpt_close_ch().
- srpt_close_ch() changes the channel state into CH_DISCONNECTING, the
queue pair state into IB_QPS_ERR and queues a zero-length write.
- Upon receipt of the zero-length write completion the channel state is
changed into CH_DISCONNECTED. This is the last work completion so once the
channel state has reached CH_DISCONNECTED it is guaranteed that the queue
pair completion handler won't be invoked again and also that the completion
handler won't call wake_up_process(ch->thread) anymore.
- That channel state change causes the polling loop in the completion thread
to stop and also triggers a call of scst_unregister_session().
- Once scst_unregister_session() has finished srpt_unreg_sess() is invoked.
- srpt_unreg_sess() destroys the CM ID and decrements the channel refcount.
- Once the channel refcount drops to zero srpt_free_ch() is invoked which
frees the queue pair and other IB resources.