Hello, James. This patch adds Documentation/scsi/scs_eh.txt. I've chosen plain text over DocBook as most other scsi docs are in plain text and it's more accessible. The document is mostly the same as previously posted. Just some rewordings, typo fixes and minor changes went in. Signed-off-by: Tejun Heo <htejun@xxxxxxxxx> diff --git a/Documentation/scsi/00-INDEX b/Documentation/scsi/00-INDEX --- a/Documentation/scsi/00-INDEX +++ b/Documentation/scsi/00-INDEX @@ -60,6 +60,8 @@ scsi.txt - short blurb on using SCSI support as a module. scsi_mid_low_api.txt - info on API between SCSI layer and low level drivers +scsi_eh.txt + - info on SCSI midlayer error handling infrastructure st.txt - info on scsi tape driver sym53c500_cs.txt diff --git a/Documentation/scsi/scsi_eh.txt b/Documentation/scsi/scsi_eh.txt new file mode 100644 --- /dev/null +++ b/Documentation/scsi/scsi_eh.txt @@ -0,0 +1,479 @@ + +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@xxxxxxxxx +11th September 2005 - : send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
