476 lines
16 KiB
Plaintext
476 lines
16 KiB
Plaintext
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SCSI EH
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======================================
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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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[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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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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discussion. The latter is used for completion and EH lists and unless
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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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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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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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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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scsi_io_completion() then notifies the block layer on
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the completed request by calling blk_end_request and
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friends or figures out what to do with the remainder
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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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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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be one of
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- BLK_EH_RESET_TIMER
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This indicates that more time is required to finish the
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command. Timer is restarted. This action is counted as a
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retry and only allowed scmd->allowed + 1(!) times. Once the
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limit is reached, action for BLK_EH_DONE is taken instead.
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- BLK_EH_DONE
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eh_timed_out() callback did not handle the command.
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Step #2 is taken.
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2. scsi_abort_command() is invoked to schedule an asynchrous abort.
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Asynchronous abort are not invoked for commands which the
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SCSI_EH_ABORT_SCHEDULED flag is set (this indicates that the command
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already had been aborted once, and this is a retry which failed),
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or when the EH deadline is expired. In these case Step #3 is taken.
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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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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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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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2. Sets SHOST_RECOVERY bit in shost->shost_state
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3. Increments shost->host_failed
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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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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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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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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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about the scmd, which currently no LLDD does, the command is still
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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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forget about - timed out scmds later.
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[2] How SCSI EH works
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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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LLDD can implement fine-grained EH callbacks and let SCSI
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midlayer drive error handling and call appropriate callbacks.
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This will be discussed further in [2-1].
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- eh_strategy_handler() callback
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This is one big callback which should perform whole error
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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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calling scsi_restart_operations(), which
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1. Checks if door locking is needed and locks door.
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2. Clears SHOST_RECOVERY shost_state bit
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3. Wakes up waiters on shost->host_wait. This occurs if someone
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calls scsi_block_when_processing_errors() on the host.
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(*QUESTION* why is it needed? All operations will be blocked
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anyway after it reaches blk queue.)
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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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[2-1-1] Overview
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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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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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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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- 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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scmds are removed from eh_work_q.
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Note that single recovery action on a scmd can recover multiple
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scmds. e.g. resetting a device recovers all failed scmds on the
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device.
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- Higher severity actions are taken iff eh_work_q is not empty after
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lower severity actions are complete.
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- EH reuses failed scmds to issue commands for recovery. For
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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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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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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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1. Error completion / time out
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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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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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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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- 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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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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[2-1-3] Flow of control
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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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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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cleared by this action.
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2. Invoke 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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SCSI transports/LLDDs automatically acquire sense data on
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command failures (autosense). Autosense is recommended for
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performance reasons and as sense information could get out of
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sync between occurrence of CHECK CONDITION and this action.
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Note that if autosense is not supported, scmd->sense_buffer
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contains invalid sense data when error-completing the scmd
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with scsi_done(). scsi_decide_disposition() always returns
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FAILED in such cases thus invoking SCSI EH. When the scmd
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reaches here, sense data is acquired and
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scsi_decide_disposition() is called again.
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1. Invoke scsi_request_sense() which issues REQUEST_SENSE
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command. If fails, no action. Note that taking no action
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causes higher-severity recovery to be taken for the scmd.
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2. Invoke scsi_decide_disposition() on the scmd
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- SUCCESS
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scmd->retries is set to scmd->allowed preventing
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scsi_eh_flush_done_q() from retrying the scmd and
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scsi_eh_finish_cmd() is invoked.
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- NEEDS_RETRY
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scsi_eh_finish_cmd() invoked
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- otherwise
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No action.
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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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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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hostt->eh_abort_handler() is invoked for each scmd. The
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handler returns SUCCESS if it has succeeded to make LLDD and
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all related hardware forget about the scmd.
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If a timedout scmd is successfully aborted and the sdev is
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either offline or ready, scsi_eh_finish_cmd() is invoked for
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the scmd. Otherwise, the scmd is left in eh_work_q for
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higher-severity actions.
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Note that both offline and ready status mean that the sdev is
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ready to process new scmds, where processing also implies
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immediate failing; thus, if a sdev is in one of the two
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states, no further recovery action is needed.
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Device readiness is tested using scsi_eh_tur() which issues
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TEST_UNIT_READY command. Note that the scmd must have been
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aborted successfully before reusing it for TEST_UNIT_READY.
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4. If !list_empty(&eh_work_q), invoke 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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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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START_STOP_UNIT command is issued w/ start=1. Note that
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as we explicitly choose error-completed scmds, it is known
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that lower layers have forgotten about the scmd and we can
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reuse it for STU.
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If STU succeeds and the sdev is either offline or ready,
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all failed scmds on the sdev are EH-finished with
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scsi_eh_finish_cmd().
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*NOTE* If hostt->eh_abort_handler() isn't implemented or
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failed, we may still have timed out scmds at this point
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and STU doesn't make lower layers forget about those
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scmds. Yet, this function EH-finish all scmds on the sdev
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if STU succeeds leaving lower layers in an inconsistent
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state. It seems that STU action should be taken only when
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a sdev has no timed out scmd.
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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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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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is used. Also, as we're not issuing SCSI commands and
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resetting clears all scmds on the sdev, there is no need
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to choose error-completed scmds.
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3. If !list_empty(&eh_work_q), invoke scsi_eh_bus_reset()
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<<scsi_eh_bus_reset>>
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hostt->eh_bus_reset_handler() is invoked for each channel
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with failed scmds. If bus reset succeeds, all failed
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scmds on all ready or offline sdevs on the channel are
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EH-finished.
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4. If !list_empty(&eh_work_q), invoke scsi_eh_host_reset()
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<<scsi_eh_host_reset>>
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This is the last resort. hostt->eh_host_reset_handler()
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is invoked. If host reset succeeds, all failed scmds on
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all ready or offline sdevs on the host are EH-finished.
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5. If !list_empty(&eh_work_q), invoke scsi_eh_offline_sdevs()
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<<scsi_eh_offline_sdevs>>
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Take all sdevs which still have unrecovered scmds offline
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and EH-finish the scmds.
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5. Invoke scsi_eh_flush_done_q().
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<<scsi_eh_flush_done_q>>
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At this point all scmds are recovered (or given up) and
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put on eh_done_q by scsi_eh_finish_cmd(). This function
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flushes eh_done_q by either retrying or notifying upper
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layer of failure of the scmds.
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[2-2] EH through transportt->eh_strategy_handler()
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transportt->eh_strategy_handler() is invoked in the place of
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scsi_unjam_host() and it is responsible for whole recovery process.
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On completion, the handler should have made lower layers forget about
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all failed scmds and either ready for new commands or offline. Also,
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it should perform SCSI EH maintenance chores to maintain integrity of
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SCSI midlayer. IOW, of the steps described in [2-1-2], all steps
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except for #1 must be implemented by eh_strategy_handler().
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[2-2-1] Pre transportt->eh_strategy_handler() SCSI midlayer conditions
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The following conditions are true on entry to the handler.
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- Each failed scmd's eh_flags field is set appropriately.
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- Each failed scmd is linked on scmd->eh_cmd_q by scmd->eh_entry.
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- SHOST_RECOVERY is set.
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- shost->host_failed == shost->host_busy
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[2-2-2] Post transportt->eh_strategy_handler() SCSI midlayer conditions
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The following conditions must be true on exit from the handler.
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- shost->host_failed is zero.
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- Each scmd is in such a state that scsi_setup_cmd_retry() on the
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scmd doesn't make any difference.
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- shost->eh_cmd_q is cleared.
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- Each scmd->eh_entry is cleared.
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- Either scsi_queue_insert() or scsi_finish_command() is called on
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each scmd. Note that the handler is free to use scmd->retries and
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->allowed to limit the number of retries.
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[2-2-3] Things to consider
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- Know that timed out scmds are still active on lower layers. Make
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lower layers forget about them before doing anything else with
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those scmds.
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- For consistency, when accessing/modifying shost data structure,
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grab shost->host_lock.
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- On completion, each failed sdev must have forgotten about all
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active scmds.
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- On completion, each failed sdev must be ready for new commands or
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offline.
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--
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Tejun Heo
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htejun@gmail.com
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11th September 2005
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