--- /dev/null
+
+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 hostt->eh_strategy_handler()
+ [2-2-1] Pre hostt->eh_strategy_handler() SCSI midlayer conditions
+ [2-2-2] Post hostt->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 SCSI midlayer 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
+does the following.
+
+ 1. Delete timeout timer. If it fails, it means that timeout timer
+ has expired and is going to finish the command. Just return.
+
+ 2. Link scmd to per-cpu scsi_done_q using scmd->en_entry
+
+ 3. 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 choirs and notify completion by
+ calling scmd->done() callback, which, for fs requests, would
+ be HLD completion callback - sd:sd_rw_intr, sr:rw_intr,
+ st:st_intr.
+
+ - 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 funciton.
+
+
+[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_timedout() callback. Return value can
+ be one of
+
+ - EH_HANDLED
+ This indicates that eh_timedout() dealt with the timeout. The
+ scmd is passed to __scsi_done() and thus linked into per-cpu
+ scsi_done_q. Normal command completion described in [1-2-1]
+ follows.
+
+ - 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 EH_NOT_HANDLED is taken instead.
+
+ *NOTE* This action is racy as the LLDD could finish the scmd
+ after the timeout has expired but before it's added back. In
+ such cases, scsi_done() would think that timeout has occurred
+ and return without doing anything. We lose completion and the
+ command will time out again.
+
+ - EH_NOT_HANDLED
+ This is the same as when eh_timedout() callback doesn't exist.
+ Step #2 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] 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_timedout() 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 dicussed 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 issueing 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 inbetween 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.
+ 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 hostt->eh_strategy_handler()
+
+ hostt->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 hostt->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 hostt->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
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