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>
This commit is contained in:
Mauro Carvalho Chehab
Martin K. Petersen
Родитель
731fc16c2f
Коммит
e513de9936
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@ -401,7 +401,7 @@ Error handling
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==============
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This chapter describes how errors are handled under libata. Readers are
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advised to read SCSI EH (Documentation/scsi/scsi_eh.txt) and ATA
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advised to read SCSI EH (Documentation/scsi/scsi_eh.rst) and ATA
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exceptions doc first.
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Origins of commands
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@ -32,5 +32,6 @@ Linux SCSI Subsystem
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ppa
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qlogicfas
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scsi-changer
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scsi_eh
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scsi_transport_srp/figures
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@ -1,35 +1,39 @@
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.. SPDX-License-Identifier: GPL-2.0
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=======
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SCSI EH
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======================================
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=======
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This document describes SCSI midlayer error handling infrastructure.
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This document describes SCSI midlayer error handling infrastructure.
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Please refer to Documentation/scsi/scsi_mid_low_api.txt for more
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information regarding SCSI midlayer.
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TABLE OF CONTENTS
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.. TABLE OF CONTENTS
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[1] How SCSI commands travel through the midlayer and to EH
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[1-1] struct scsi_cmnd
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[1-2] How do scmd's get completed?
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[1-2-1] Completing a scmd w/ scsi_done
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[1-2-2] Completing a scmd w/ timeout
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[1-3] How EH takes over
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[2] How SCSI EH works
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[2-1] EH through fine-grained callbacks
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[2-1-1] Overview
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[2-1-2] Flow of scmds through EH
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[2-1-3] Flow of control
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[2-2] EH through transportt->eh_strategy_handler()
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[2-2-1] Pre transportt->eh_strategy_handler() SCSI midlayer conditions
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[2-2-2] Post transportt->eh_strategy_handler() SCSI midlayer conditions
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[2-2-3] Things to consider
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[1] How SCSI commands travel through the midlayer and to EH
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[1-1] struct scsi_cmnd
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[1-2] How do scmd's get completed?
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[1-2-1] Completing a scmd w/ scsi_done
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[1-2-2] Completing a scmd w/ timeout
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[1-3] How EH takes over
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[2] How SCSI EH works
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[2-1] EH through fine-grained callbacks
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[2-1-1] Overview
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[2-1-2] Flow of scmds through EH
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[2-1-3] Flow of control
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[2-2] EH through transportt->eh_strategy_handler()
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[2-2-1] Pre transportt->eh_strategy_handler() SCSI midlayer conditions
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[2-2-2] Post transportt->eh_strategy_handler() SCSI midlayer conditions
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[2-2-3] Things to consider
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[1] How SCSI commands travel through the midlayer and to EH
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1. How SCSI commands travel through the midlayer and to EH
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==========================================================
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[1-1] struct scsi_cmnd
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1.1 struct scsi_cmnd
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--------------------
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Each SCSI command is represented with struct scsi_cmnd (== scmd). A
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Each SCSI command is represented with struct scsi_cmnd (== scmd). A
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scmd has two list_head's to link itself into lists. The two are
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scmd->list and scmd->eh_entry. The former is used for free list or
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per-device allocated scmd list and not of much interest to this EH
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@ -38,25 +42,28 @@ otherwise stated scmds are always linked using scmd->eh_entry in this
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discussion.
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[1-2] How do scmd's get completed?
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1.2 How do scmd's get completed?
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--------------------------------
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Once LLDD gets hold of a scmd, either the LLDD will complete the
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Once LLDD gets hold of a scmd, either the LLDD will complete the
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command by calling scsi_done callback passed from midlayer when
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invoking hostt->queuecommand() or the block layer will time it out.
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[1-2-1] Completing a scmd w/ scsi_done
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1.2.1 Completing a scmd w/ scsi_done
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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For all non-EH commands, scsi_done() is the completion callback. It
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For all non-EH commands, scsi_done() is the completion callback. It
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just calls blk_complete_request() to delete the block layer timer and
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raise SCSI_SOFTIRQ
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SCSI_SOFTIRQ handler scsi_softirq calls scsi_decide_disposition() to
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SCSI_SOFTIRQ handler scsi_softirq calls scsi_decide_disposition() to
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determine what to do with the command. scsi_decide_disposition()
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looks at the scmd->result value and sense data to determine what to do
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with the command.
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- SUCCESS
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scsi_finish_command() is invoked for the command. The
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function does some maintenance chores and then calls
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scsi_io_completion() to finish the I/O.
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@ -66,17 +73,21 @@ with the command.
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of the data in case of an error.
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- NEEDS_RETRY
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- ADD_TO_MLQUEUE
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scmd is requeued to blk queue.
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- otherwise
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scsi_eh_scmd_add(scmd) is invoked for the command. See
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[1-3] for details of this function.
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[1-2-2] Completing a scmd w/ timeout
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1.2.2 Completing a scmd w/ timeout
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The timeout handler is scsi_times_out(). When a timeout occurs, this
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The timeout handler is scsi_times_out(). When a timeout occurs, this
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function
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1. invokes optional hostt->eh_timed_out() callback. Return value can
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@ -101,18 +112,21 @@ function
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3. scsi_eh_scmd_add(scmd, SCSI_EH_CANCEL_CMD) is invoked for the
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command. See [1-4] for more information.
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[1-3] Asynchronous command aborts
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1.3 Asynchronous command aborts
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-------------------------------
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After a timeout occurs a command abort is scheduled from
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scsi_abort_command(). If the abort is successful the command
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will either be retried (if the number of retries is not exhausted)
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or terminated with DID_TIME_OUT.
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Otherwise scsi_eh_scmd_add() is invoked for the command.
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See [1-4] for more information.
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[1-4] How EH takes over
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1.4 How EH takes over
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---------------------
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scmds enter EH via scsi_eh_scmd_add(), which does the following.
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scmds enter EH via scsi_eh_scmd_add(), which does the following.
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1. Links scmd->eh_entry to shost->eh_cmd_q
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@ -122,19 +136,19 @@ function
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4. Wakes up SCSI EH thread if shost->host_busy == shost->host_failed
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As can be seen above, once any scmd is added to shost->eh_cmd_q,
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As can be seen above, once any scmd is added to shost->eh_cmd_q,
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SHOST_RECOVERY shost_state bit is turned on. This prevents any new
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scmd to be issued from blk queue to the host; eventually, all scmds on
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the host either complete normally, fail and get added to eh_cmd_q, or
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time out and get added to shost->eh_cmd_q.
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If all scmds either complete or fail, the number of in-flight scmds
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If all scmds either complete or fail, the number of in-flight scmds
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becomes equal to the number of failed scmds - i.e. shost->host_busy ==
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shost->host_failed. This wakes up SCSI EH thread. So, once woken up,
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SCSI EH thread can expect that all in-flight commands have failed and
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are linked on shost->eh_cmd_q.
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Note that this does not mean lower layers are quiescent. If a LLDD
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Note that this does not mean lower layers are quiescent. If a LLDD
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completed a scmd with error status, the LLDD and lower layers are
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assumed to forget about the scmd at that point. However, if a scmd
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has timed out, unless hostt->eh_timed_out() made lower layers forget
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@ -143,13 +157,14 @@ active as long as lower layers are concerned and completion could
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occur at any time. Of course, all such completions are ignored as the
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timer has already expired.
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We'll talk about how SCSI EH takes actions to abort - make LLDD
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We'll talk about how SCSI EH takes actions to abort - make LLDD
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forget about - timed out scmds later.
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[2] How SCSI EH works
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2. How SCSI EH works
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====================
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LLDD's can implement SCSI EH actions in one of the following two
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LLDD's can implement SCSI EH actions in one of the following two
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ways.
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- Fine-grained EH callbacks
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@ -162,7 +177,7 @@ ways.
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handling. As such, it should do all chores the SCSI midlayer
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performs during recovery. This will be discussed in [2-2].
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Once recovery is complete, SCSI EH resumes normal operation by
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Once recovery is complete, SCSI EH resumes normal operation by
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calling scsi_restart_operations(), which
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1. Checks if door locking is needed and locks door.
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@ -177,34 +192,38 @@ calling scsi_restart_operations(), which
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4. Kicks queues in all devices on the host in the asses
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[2-1] EH through fine-grained callbacks
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2.1 EH through fine-grained callbacks
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-------------------------------------
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[2-1-1] Overview
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2.1.1 Overview
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^^^^^^^^^^^^^^
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If eh_strategy_handler() is not present, SCSI midlayer takes charge
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If eh_strategy_handler() is not present, SCSI midlayer takes charge
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of driving error handling. EH's goals are two - make LLDD, host and
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device forget about timed out scmds and make them ready for new
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commands. A scmd is said to be recovered if the scmd is forgotten by
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lower layers and lower layers are ready to process or fail the scmd
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again.
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To achieve these goals, EH performs recovery actions with increasing
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To achieve these goals, EH performs recovery actions with increasing
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severity. Some actions are performed by issuing SCSI commands and
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others are performed by invoking one of the following fine-grained
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hostt EH callbacks. Callbacks may be omitted and omitted ones are
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considered to fail always.
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int (* eh_abort_handler)(struct scsi_cmnd *);
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int (* eh_device_reset_handler)(struct scsi_cmnd *);
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int (* eh_bus_reset_handler)(struct scsi_cmnd *);
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int (* eh_host_reset_handler)(struct scsi_cmnd *);
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::
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Higher-severity actions are taken only when lower-severity actions
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int (* eh_abort_handler)(struct scsi_cmnd *);
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int (* eh_device_reset_handler)(struct scsi_cmnd *);
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int (* eh_bus_reset_handler)(struct scsi_cmnd *);
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int (* eh_host_reset_handler)(struct scsi_cmnd *);
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Higher-severity actions are taken only when lower-severity actions
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cannot recover some of failed scmds. Also, note that failure of the
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highest-severity action means EH failure and results in offlining of
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all unrecovered devices.
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During recovery, the following rules are followed
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During recovery, the following rules are followed
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- Recovery actions are performed on failed scmds on the to do list,
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eh_work_q. If a recovery action succeeds for a scmd, recovered
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@ -221,58 +240,72 @@ all unrecovered devices.
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timed-out scmds, SCSI EH ensures that LLDD forgets about a scmd
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before reusing it for EH commands.
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When a scmd is recovered, the scmd is moved from eh_work_q to EH
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When a scmd is recovered, the scmd is moved from eh_work_q to EH
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local eh_done_q using scsi_eh_finish_cmd(). After all scmds are
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recovered (eh_work_q is empty), scsi_eh_flush_done_q() is invoked to
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either retry or error-finish (notify upper layer of failure) recovered
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scmds.
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scmds are retried iff its sdev is still online (not offlined during
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scmds are retried iff its sdev is still online (not offlined during
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EH), REQ_FAILFAST is not set and ++scmd->retries is less than
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scmd->allowed.
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[2-1-2] Flow of scmds through EH
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2.1.2 Flow of scmds through EH
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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1. Error completion / time out
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ACTION: scsi_eh_scmd_add() is invoked for scmd
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:ACTION: scsi_eh_scmd_add() is invoked for scmd
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- add scmd to shost->eh_cmd_q
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- set SHOST_RECOVERY
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- shost->host_failed++
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LOCKING: shost->host_lock
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:LOCKING: shost->host_lock
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2. EH starts
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ACTION: move all scmds to EH's local eh_work_q. shost->eh_cmd_q
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is cleared.
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LOCKING: shost->host_lock (not strictly necessary, just for
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:ACTION: move all scmds to EH's local eh_work_q. shost->eh_cmd_q
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is cleared.
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:LOCKING: shost->host_lock (not strictly necessary, just for
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consistency)
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3. scmd recovered
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ACTION: scsi_eh_finish_cmd() is invoked to EH-finish scmd
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:ACTION: scsi_eh_finish_cmd() is invoked to EH-finish scmd
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- scsi_setup_cmd_retry()
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- move from local eh_work_q to local eh_done_q
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LOCKING: none
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CONCURRENCY: at most one thread per separate eh_work_q to
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keep queue manipulation lockless
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:LOCKING: none
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:CONCURRENCY: at most one thread per separate eh_work_q to
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keep queue manipulation lockless
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4. EH completes
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ACTION: scsi_eh_flush_done_q() retries scmds or notifies upper
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layer of failure. May be called concurrently but must have
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a no more than one thread per separate eh_work_q to
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manipulate the queue locklessly
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- scmd is removed from eh_done_q and scmd->eh_entry is cleared
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- if retry is necessary, scmd is requeued using
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scsi_queue_insert()
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- otherwise, scsi_finish_command() is invoked for scmd
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- zero shost->host_failed
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LOCKING: queue or finish function performs appropriate locking
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:ACTION: scsi_eh_flush_done_q() retries scmds or notifies upper
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layer of failure. May be called concurrently but must have
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a no more than one thread per separate eh_work_q to
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manipulate the queue locklessly
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- scmd is removed from eh_done_q and scmd->eh_entry is cleared
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- if retry is necessary, scmd is requeued using
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scsi_queue_insert()
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- otherwise, scsi_finish_command() is invoked for scmd
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- zero shost->host_failed
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:LOCKING: queue or finish function performs appropriate locking
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[2-1-3] Flow of control
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2.1.3 Flow of control
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^^^^^^^^^^^^^^^^^^^^^^
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EH through fine-grained callbacks start from scsi_unjam_host().
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<<scsi_unjam_host>>
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``scsi_unjam_host``
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1. Lock shost->host_lock, splice_init shost->eh_cmd_q into local
|
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eh_work_q and unlock host_lock. Note that shost->eh_cmd_q is
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@ -280,7 +313,7 @@ scmd->allowed.
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2. Invoke scsi_eh_get_sense.
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<<scsi_eh_get_sense>>
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``scsi_eh_get_sense``
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This action is taken for each error-completed
|
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(!SCSI_EH_CANCEL_CMD) commands without valid sense data. Most
|
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@ -315,7 +348,7 @@ scmd->allowed.
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3. If !list_empty(&eh_work_q), invoke scsi_eh_abort_cmds().
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<<scsi_eh_abort_cmds>>
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``scsi_eh_abort_cmds``
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|
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This action is taken for each timed out command when
|
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no_async_abort is enabled in the host template.
|
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@ -339,14 +372,14 @@ scmd->allowed.
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||||
4. If !list_empty(&eh_work_q), invoke scsi_eh_ready_devs()
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||||
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||||
<<scsi_eh_ready_devs>>
|
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``scsi_eh_ready_devs``
|
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This function takes four increasingly more severe measures to
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make failed sdevs ready for new commands.
|
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|
||||
1. Invoke scsi_eh_stu()
|
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<<scsi_eh_stu>>
|
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``scsi_eh_stu``
|
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For each sdev which has failed scmds with valid sense data
|
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of which scsi_check_sense()'s verdict is FAILED,
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@ -369,7 +402,7 @@ scmd->allowed.
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2. If !list_empty(&eh_work_q), invoke scsi_eh_bus_device_reset().
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<<scsi_eh_bus_device_reset>>
|
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``scsi_eh_bus_device_reset``
|
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This action is very similar to scsi_eh_stu() except that,
|
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instead of issuing STU, hostt->eh_device_reset_handler()
|
||||
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@ -379,7 +412,7 @@ scmd->allowed.
|
|||
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||||
3. If !list_empty(&eh_work_q), invoke scsi_eh_bus_reset()
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||||
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||||
<<scsi_eh_bus_reset>>
|
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``scsi_eh_bus_reset``
|
||||
|
||||
hostt->eh_bus_reset_handler() is invoked for each channel
|
||||
with failed scmds. If bus reset succeeds, all failed
|
||||
|
|
@ -388,7 +421,7 @@ scmd->allowed.
|
|||
|
||||
4. If !list_empty(&eh_work_q), invoke scsi_eh_host_reset()
|
||||
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||||
<<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
|
||||
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@ -396,14 +429,14 @@ scmd->allowed.
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|||
|
||||
5. If !list_empty(&eh_work_q), invoke scsi_eh_offline_sdevs()
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||||
<<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>>
|
||||
``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
|
||||
|
|
@ -411,9 +444,10 @@ scmd->allowed.
|
|||
layer of failure of the scmds.
|
||||
|
||||
|
||||
[2-2] EH through transportt->eh_strategy_handler()
|
||||
2.2 EH through transportt->eh_strategy_handler()
|
||||
------------------------------------------------
|
||||
|
||||
transportt->eh_strategy_handler() is invoked in the place of
|
||||
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,
|
||||
|
|
@ -422,7 +456,8 @@ 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
|
||||
2.2.1 Pre transportt->eh_strategy_handler() SCSI midlayer conditions
|
||||
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
||||
|
||||
The following conditions are true on entry to the handler.
|
||||
|
||||
|
|
@ -435,7 +470,8 @@ except for #1 must be implemented by eh_strategy_handler().
|
|||
- shost->host_failed == shost->host_busy
|
||||
|
||||
|
||||
[2-2-2] Post transportt->eh_strategy_handler() SCSI midlayer conditions
|
||||
2.2.2 Post transportt->eh_strategy_handler() SCSI midlayer conditions
|
||||
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
||||
|
||||
The following conditions must be true on exit from the handler.
|
||||
|
||||
|
|
@ -453,7 +489,8 @@ except for #1 must be implemented by eh_strategy_handler().
|
|||
->allowed to limit the number of retries.
|
||||
|
||||
|
||||
[2-2-3] Things to consider
|
||||
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
|
||||
|
|
@ -469,7 +506,7 @@ except for #1 must be implemented by eh_strategy_handler().
|
|||
offline.
|
||||
|
||||
|
||||
--
|
||||
Tejun Heo
|
||||
htejun@gmail.com
|
||||
|
||||
11th September 2005
|
||||
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