scsi: docs: convert scsi_eh.txt to ReST

Link: https://lore.kernel.org/r/300314197f2e6a3258200711e825aa04c9e8ceaf.1583136624.git.mchehab+huawei@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

1 files changed, 512 insertions, 0 deletions

diff --git a/Documentation/scsi/scsi_eh.rst b/Documentation/scsi/scsi_eh.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+=======
+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) 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_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_DONE is taken instead.
+
+    - BLK_EH_DONE
+        eh_timed_out() callback did not handle the command.
+	Step #2 is taken.
+
+ 2. scsi_abort_command() is invoked to schedule an asynchrous abort.
+    Asynchronous abort are not invoked for commands which the
+    SCSI_EH_ABORT_SCHEDULED flag is set (this indicates that the command
+    already had been aborted once, and this is a retry which failed),
+    or when the EH deadline is expired. In these case Step #3 is taken.
+
+ 3. scsi_eh_scmd_add(scmd, SCSI_EH_CANCEL_CMD) is invoked for the
+    command.  See [1-4] 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. Links scmd->eh_entry to shost->eh_cmd_q
+
+ 2. Sets SHOST_RECOVERY bit in shost->shost_state
+
+ 3. Increments shost->host_failed
+
+ 4. 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 chores the 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
+
+	- 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
+
+	- scsi_setup_cmd_retry()
+	- move from local eh_work_q to local eh_done_q
+
+    :LOCKING: none
+
+    :CONCURRENCY: at most one thread per separate eh_work_q to
+		  keep queue manipulation lockless
+
+ 4. EH completes
+
+    :ACTION: scsi_eh_flush_done_q() retries scmds or notifies upper
+	     layer of failure. May be called concurrently but must have
+	     a no more than one thread per separate eh_work_q to
+	     manipulate the queue locklessly
+
+	     - 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
+	     - zero shost->host_failed
+
+    :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 chores 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 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