SCSI EH
  ======================================
  
   This document describes SCSI midlayer error handling infrastructure.
  Please refer to Documentation/scsi/scsi_mid_low_api.txt for more
  information regarding SCSI midlayer.
  
  TABLE OF CONTENTS
  
  [1] How SCSI commands travel through the midlayer and to EH
      [1-1] struct scsi_cmnd
      [1-2] How do scmd's get completed?
  	[1-2-1] Completing a scmd w/ scsi_done
  	[1-2-2] Completing a scmd w/ timeout
      [1-3] How EH takes over
  [2] How SCSI EH works
      [2-1] EH through fine-grained callbacks
  	[2-1-1] Overview
  	[2-1-2] Flow of scmds through EH
  	[2-1-3] Flow of control
      [2-2] EH through transportt->eh_strategy_handler()
  	[2-2-1] Pre transportt->eh_strategy_handler() SCSI midlayer conditions
  	[2-2-2] Post transportt->eh_strategy_handler() SCSI midlayer conditions
  	[2-2-3] Things to consider
  
  
  [1] How SCSI commands travel through the midlayer and to EH
  
  [1-1] struct scsi_cmnd
  
   Each SCSI command is represented with struct scsi_cmnd (== scmd).  A
  scmd has two list_head's to link itself into lists.  The two are
  scmd->list and scmd->eh_entry.  The former is used for free list or
  per-device allocated scmd list and not of much interest to this EH
  discussion.  The latter is used for completion and EH lists and unless
  otherwise stated scmds are always linked using scmd->eh_entry in this
  discussion.
  
  
  [1-2] How do scmd's get completed?
  
   Once LLDD gets hold of a scmd, either the LLDD will complete the
  command by calling scsi_done callback passed from midlayer when
  invoking hostt->queuecommand() or the block layer will time it out.
  
  
  [1-2-1] Completing a scmd w/ scsi_done
  
   For all non-EH commands, scsi_done() is the completion callback.  It
  just calls blk_complete_request() to delete the block layer timer and
  raise SCSI_SOFTIRQ
  
   SCSI_SOFTIRQ handler scsi_softirq calls scsi_decide_disposition() to
  determine what to do with the command.  scsi_decide_disposition()
  looks at the scmd->result value and sense data to determine what to do
  with the command.
  
   - SUCCESS
  	scsi_finish_command() is invoked for the command.  The
  	function does some maintenance chores and then calls
  	scsi_io_completion() to finish the I/O.
  	scsi_io_completion() then notifies the block layer on
  	the completed request by calling blk_end_request and
  	friends or figures out what to do with the remainder
  	of the data in case of an error.
  
   - NEEDS_RETRY
   - ADD_TO_MLQUEUE
  	scmd is requeued to blk queue.
  
   - otherwise
  	scsi_eh_scmd_add(scmd, 0) is invoked for the command.  See
  	[1-3] for details of this function.
  
  
  [1-2-2] Completing a scmd w/ timeout
  
   The timeout handler is scsi_times_out().  When a timeout occurs, this
  function
  
   1. invokes optional hostt->eh_timed_out() callback.  Return value can
      be one of
  
      - BLK_EH_HANDLED
  	This indicates that eh_timed_out() dealt with the timeout.
  	The command is passed back to the block layer and completed
  	via __blk_complete_requests().
  
  	*NOTE* After returning BLK_EH_HANDLED the SCSI layer is
  	assumed to be finished with the command, and no other
  	functions from the SCSI layer will be called. So this
  	should typically only be returned if the eh_timed_out()
  	handler raced with normal completion.
  
      - BLK_EH_RESET_TIMER
  	This indicates that more time is required to finish the
  	command.  Timer is restarted.  This action is counted as a
  	retry and only allowed scmd->allowed + 1(!) times.  Once the
  	limit is reached, action for BLK_EH_NOT_HANDLED is taken instead.
  
      - BLK_EH_NOT_HANDLED
          eh_timed_out() callback did not handle the command.
  	Step #2 is taken.
  
   2. If the host supports asynchronous completion (as indicated by the
      no_async_abort setting in the host template) scsi_abort_command()
      is invoked to schedule an asynchrous abort. If that fails
      Step #3 is taken.
  
   2. scsi_eh_scmd_add(scmd, SCSI_EH_CANCEL_CMD) is invoked for the
      command.  See [1-3] for more information.
  
  [1-3] Asynchronous command aborts
  
   After a timeout occurs a command abort is scheduled from
   scsi_abort_command(). If the abort is successful the command
   will either be retried (if the number of retries is not exhausted)
   or terminated with DID_TIME_OUT.
   Otherwise scsi_eh_scmd_add() is invoked for the command.
   See [1-4] for more information.
  
  [1-4] How EH takes over
  
   scmds enter EH via scsi_eh_scmd_add(), which does the following.
  
   1. Turns on scmd->eh_eflags as requested.  It's 0 for error
      completions and SCSI_EH_CANCEL_CMD for timeouts.
  
   2. Links scmd->eh_entry to shost->eh_cmd_q
  
   3. Sets SHOST_RECOVERY bit in shost->shost_state
  
   4. Increments shost->host_failed
  
   5. Wakes up SCSI EH thread if shost->host_busy == shost->host_failed
  
   As can be seen above, once any scmd is added to shost->eh_cmd_q,
  SHOST_RECOVERY shost_state bit is turned on.  This prevents any new
  scmd to be issued from blk queue to the host; eventually, all scmds on
  the host either complete normally, fail and get added to eh_cmd_q, or
  time out and get added to shost->eh_cmd_q.
  
   If all scmds either complete or fail, the number of in-flight scmds
  becomes equal to the number of failed scmds - i.e. shost->host_busy ==
  shost->host_failed.  This wakes up SCSI EH thread.  So, once woken up,
  SCSI EH thread can expect that all in-flight commands have failed and
  are linked on shost->eh_cmd_q.
  
   Note that this does not mean lower layers are quiescent.  If a LLDD
  completed a scmd with error status, the LLDD and lower layers are
  assumed to forget about the scmd at that point.  However, if a scmd
  has timed out, unless hostt->eh_timed_out() made lower layers forget
  about the scmd, which currently no LLDD does, the command is still
  active as long as lower layers are concerned and completion could
  occur at any time.  Of course, all such completions are ignored as the
  timer has already expired.
  
   We'll talk about how SCSI EH takes actions to abort - make LLDD
  forget about - timed out scmds later.
  
  
  [2] How SCSI EH works
  
   LLDD's can implement SCSI EH actions in one of the following two
  ways.
  
   - Fine-grained EH callbacks
  	LLDD can implement fine-grained EH callbacks and let SCSI
  	midlayer drive error handling and call appropriate callbacks.
  	This will be discussed further in [2-1].
  
   - eh_strategy_handler() callback
  	This is one big callback which should perform whole error
  	handling.  As such, it should do all choirs SCSI midlayer
  	performs during recovery.  This will be discussed in [2-2].
  
   Once recovery is complete, SCSI EH resumes normal operation by
  calling scsi_restart_operations(), which
  
   1. Checks if door locking is needed and locks door.
  
   2. Clears SHOST_RECOVERY shost_state bit
  
   3. Wakes up waiters on shost->host_wait.  This occurs if someone
      calls scsi_block_when_processing_errors() on the host.
      (*QUESTION* why is it needed?  All operations will be blocked
      anyway after it reaches blk queue.)
  
   4. Kicks queues in all devices on the host in the asses
  
  
  [2-1] EH through fine-grained callbacks
  
  [2-1-1] Overview
  
   If eh_strategy_handler() is not present, SCSI midlayer takes charge
  of driving error handling.  EH's goals are two - make LLDD, host and
  device forget about timed out scmds and make them ready for new
  commands.  A scmd is said to be recovered if the scmd is forgotten by
  lower layers and lower layers are ready to process or fail the scmd
  again.
  
   To achieve these goals, EH performs recovery actions with increasing
  severity.  Some actions are performed by issuing SCSI commands and
  others are performed by invoking one of the following fine-grained
  hostt EH callbacks.  Callbacks may be omitted and omitted ones are
  considered to fail always.
  
  int (* eh_abort_handler)(struct scsi_cmnd *);
  int (* eh_device_reset_handler)(struct scsi_cmnd *);
  int (* eh_bus_reset_handler)(struct scsi_cmnd *);
  int (* eh_host_reset_handler)(struct scsi_cmnd *);
  
   Higher-severity actions are taken only when lower-severity actions
  cannot recover some of failed scmds.  Also, note that failure of the
  highest-severity action means EH failure and results in offlining of
  all unrecovered devices.
  
   During recovery, the following rules are followed
  
   - Recovery actions are performed on failed scmds on the to do list,
     eh_work_q.  If a recovery action succeeds for a scmd, recovered
     scmds are removed from eh_work_q.
  
     Note that single recovery action on a scmd can recover multiple
     scmds.  e.g. resetting a device recovers all failed scmds on the
     device.
  
   - Higher severity actions are taken iff eh_work_q is not empty after
     lower severity actions are complete.
  
   - EH reuses failed scmds to issue commands for recovery.  For
     timed-out scmds, SCSI EH ensures that LLDD forgets about a scmd
     before reusing it for EH commands.
  
   When a scmd is recovered, the scmd is moved from eh_work_q to EH
  local eh_done_q using scsi_eh_finish_cmd().  After all scmds are
  recovered (eh_work_q is empty), scsi_eh_flush_done_q() is invoked to
  either retry or error-finish (notify upper layer of failure) recovered
  scmds.
  
   scmds are retried iff its sdev is still online (not offlined during
  EH), REQ_FAILFAST is not set and ++scmd->retries is less than
  scmd->allowed.
  
  
  [2-1-2] Flow of scmds through EH
  
   1. Error completion / time out
      ACTION: scsi_eh_scmd_add() is invoked for scmd
  	- set scmd->eh_eflags
  	- add scmd to shost->eh_cmd_q
  	- set SHOST_RECOVERY
  	- shost->host_failed++
      LOCKING: shost->host_lock
  
   2. EH starts
      ACTION: move all scmds to EH's local eh_work_q.  shost->eh_cmd_q
  	    is cleared.
      LOCKING: shost->host_lock (not strictly necessary, just for
               consistency)
  
   3. scmd recovered
      ACTION: scsi_eh_finish_cmd() is invoked to EH-finish scmd
  	- shost->host_failed--
  	- clear scmd->eh_eflags
  	- scsi_setup_cmd_retry()
  	- move from local eh_work_q to local eh_done_q
      LOCKING: none
  
   4. EH completes
      ACTION: scsi_eh_flush_done_q() retries scmds or notifies upper
  	    layer of failure.
  	- scmd is removed from eh_done_q and scmd->eh_entry is cleared
  	- if retry is necessary, scmd is requeued using
            scsi_queue_insert()
  	- otherwise, scsi_finish_command() is invoked for scmd
      LOCKING: queue or finish function performs appropriate locking
  
  
  [2-1-3] Flow of control
  
   EH through fine-grained callbacks start from scsi_unjam_host().
  
  <<scsi_unjam_host>>
  
      1. Lock shost->host_lock, splice_init shost->eh_cmd_q into local
         eh_work_q and unlock host_lock.  Note that shost->eh_cmd_q is
         cleared by this action.
  
      2. Invoke scsi_eh_get_sense.
  
      <<scsi_eh_get_sense>>
  
  	This action is taken for each error-completed
  	(!SCSI_EH_CANCEL_CMD) commands without valid sense data.  Most
  	SCSI transports/LLDDs automatically acquire sense data on
  	command failures (autosense).  Autosense is recommended for
  	performance reasons and as sense information could get out of
  	sync between occurrence of CHECK CONDITION and this action.
  
  	Note that if autosense is not supported, scmd->sense_buffer
  	contains invalid sense data when error-completing the scmd
  	with scsi_done().  scsi_decide_disposition() always returns
  	FAILED in such cases thus invoking SCSI EH.  When the scmd
  	reaches here, sense data is acquired and
  	scsi_decide_disposition() is called again.
  
  	1. Invoke scsi_request_sense() which issues REQUEST_SENSE
             command.  If fails, no action.  Note that taking no action
             causes higher-severity recovery to be taken for the scmd.
  
  	2. Invoke scsi_decide_disposition() on the scmd
  
  	   - SUCCESS
  		scmd->retries is set to scmd->allowed preventing
  		scsi_eh_flush_done_q() from retrying the scmd and
  		scsi_eh_finish_cmd() is invoked.
  
  	   - NEEDS_RETRY
  		scsi_eh_finish_cmd() invoked
  
  	   - otherwise
  		No action.
  
      3. If !list_empty(&eh_work_q), invoke scsi_eh_abort_cmds().
  
      <<scsi_eh_abort_cmds>>
  
  	This action is taken for each timed out command when
  	no_async_abort is enabled in the host template.
  	hostt->eh_abort_handler() is invoked for each scmd.  The
  	handler returns SUCCESS if it has succeeded to make LLDD and
  	all related hardware forget about the scmd.
  
  	If a timedout scmd is successfully aborted and the sdev is
  	either offline or ready, scsi_eh_finish_cmd() is invoked for
  	the scmd.  Otherwise, the scmd is left in eh_work_q for
  	higher-severity actions.
  
  	Note that both offline and ready status mean that the sdev is
  	ready to process new scmds, where processing also implies
  	immediate failing; thus, if a sdev is in one of the two
  	states, no further recovery action is needed.
  
  	Device readiness is tested using scsi_eh_tur() which issues
  	TEST_UNIT_READY command.  Note that the scmd must have been
  	aborted successfully before reusing it for TEST_UNIT_READY.
  
      4. If !list_empty(&eh_work_q), invoke scsi_eh_ready_devs()
  
      <<scsi_eh_ready_devs>>
  
  	This function takes four increasingly more severe measures to
  	make failed sdevs ready for new commands.
  
  	1. Invoke scsi_eh_stu()
  
  	<<scsi_eh_stu>>
  
  	    For each sdev which has failed scmds with valid sense data
  	    of which scsi_check_sense()'s verdict is FAILED,
  	    START_STOP_UNIT command is issued w/ start=1.  Note that
  	    as we explicitly choose error-completed scmds, it is known
  	    that lower layers have forgotten about the scmd and we can
  	    reuse it for STU.
  
  	    If STU succeeds and the sdev is either offline or ready,
  	    all failed scmds on the sdev are EH-finished with
  	    scsi_eh_finish_cmd().
  
  	    *NOTE* If hostt->eh_abort_handler() isn't implemented or
  	    failed, we may still have timed out scmds at this point
  	    and STU doesn't make lower layers forget about those
  	    scmds.  Yet, this function EH-finish all scmds on the sdev
  	    if STU succeeds leaving lower layers in an inconsistent
  	    state.  It seems that STU action should be taken only when
  	    a sdev has no timed out scmd.
  
  	2. If !list_empty(&eh_work_q), invoke scsi_eh_bus_device_reset().
  
  	<<scsi_eh_bus_device_reset>>
  
  	    This action is very similar to scsi_eh_stu() except that,
  	    instead of issuing STU, hostt->eh_device_reset_handler()
  	    is used.  Also, as we're not issuing SCSI commands and
  	    resetting clears all scmds on the sdev, there is no need
  	    to choose error-completed scmds.
  
  	3. If !list_empty(&eh_work_q), invoke scsi_eh_bus_reset()
  
  	<<scsi_eh_bus_reset>>
  
  	    hostt->eh_bus_reset_handler() is invoked for each channel
  	    with failed scmds.  If bus reset succeeds, all failed
  	    scmds on all ready or offline sdevs on the channel are
  	    EH-finished.
  
  	4. If !list_empty(&eh_work_q), invoke scsi_eh_host_reset()
  
  	<<scsi_eh_host_reset>>
  
  	    This is the last resort.  hostt->eh_host_reset_handler()
  	    is invoked.  If host reset succeeds, all failed scmds on
  	    all ready or offline sdevs on the host are EH-finished.
  
  	5. If !list_empty(&eh_work_q), invoke scsi_eh_offline_sdevs()
  
  	<<scsi_eh_offline_sdevs>>
  
  	    Take all sdevs which still have unrecovered scmds offline
  	    and EH-finish the scmds.
  
      5. Invoke scsi_eh_flush_done_q().
  
  	<<scsi_eh_flush_done_q>>
  
  	    At this point all scmds are recovered (or given up) and
  	    put on eh_done_q by scsi_eh_finish_cmd().  This function
  	    flushes eh_done_q by either retrying or notifying upper
  	    layer of failure of the scmds.
  
  
  [2-2] EH through transportt->eh_strategy_handler()
  
   transportt->eh_strategy_handler() is invoked in the place of
  scsi_unjam_host() and it is responsible for whole recovery process.
  On completion, the handler should have made lower layers forget about
  all failed scmds and either ready for new commands or offline.  Also,
  it should perform SCSI EH maintenance choirs to maintain integrity of
  SCSI midlayer.  IOW, of the steps described in [2-1-2], all steps
  except for #1 must be implemented by eh_strategy_handler().
  
  
  [2-2-1] Pre transportt->eh_strategy_handler() SCSI midlayer conditions
  
   The following conditions are true on entry to the handler.
  
   - Each failed scmd's eh_flags field is set appropriately.
  
   - Each failed scmd is linked on scmd->eh_cmd_q by scmd->eh_entry.
  
   - SHOST_RECOVERY is set.
  
   - shost->host_failed == shost->host_busy
  
  
  [2-2-2] Post transportt->eh_strategy_handler() SCSI midlayer conditions
  
   The following conditions must be true on exit from the handler.
  
   - shost->host_failed is zero.
  
   - Each scmd's eh_eflags field is cleared.
  
   - Each scmd is in such a state that scsi_setup_cmd_retry() on the
     scmd doesn't make any difference.
  
   - shost->eh_cmd_q is cleared.
  
   - Each scmd->eh_entry is cleared.
  
   - Either scsi_queue_insert() or scsi_finish_command() is called on
     each scmd.  Note that the handler is free to use scmd->retries and
     ->allowed to limit the number of retries.
  
  
  [2-2-3] Things to consider
  
   - Know that timed out scmds are still active on lower layers.  Make
     lower layers forget about them before doing anything else with
     those scmds.
  
   - For consistency, when accessing/modifying shost data structure,
     grab shost->host_lock.
  
   - On completion, each failed sdev must have forgotten about all
     active scmds.
  
   - On completion, each failed sdev must be ready for new commands or
     offline.
  
  
  --
  Tejun Heo
  htejun@gmail.com
  11th September 2005