Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.
1 2 SCSI EH 3 ====================================== 4 5 This document describes SCSI midlayer error handling infrastructure. 6 Please refer to Documentation/scsi/scsi_mid_low_api.txt for more 7 information regarding SCSI midlayer. 8 9 TABLE OF CONTENTS 10 11  How SCSI commands travel through the midlayer and to EH 12 [1-1] struct scsi_cmnd 13 [1-2] How do scmd's get completed? 14 [1-2-1] Completing a scmd w/ scsi_done 15 [1-2-2] Completing a scmd w/ timeout 16 [1-3] How EH takes over 17  How SCSI EH works 18 [2-1] EH through fine-grained callbacks 19 [2-1-1] Overview 20 [2-1-2] Flow of scmds through EH 21 [2-1-3] Flow of control 22 [2-2] EH through transportt->eh_strategy_handler() 23 [2-2-1] Pre transportt->eh_strategy_handler() SCSI midlayer conditions 24 [2-2-2] Post transportt->eh_strategy_handler() SCSI midlayer conditions 25 [2-2-3] Things to consider 26 27 28  How SCSI commands travel through the midlayer and to EH 29 30 [1-1] struct scsi_cmnd 31 32 Each SCSI command is represented with struct scsi_cmnd (== scmd). A 33 scmd has two list_head's to link itself into lists. The two are 34 scmd->list and scmd->eh_entry. The former is used for free list or 35 per-device allocated scmd list and not of much interest to this EH 36 discussion. The latter is used for completion and EH lists and unless 37 otherwise stated scmds are always linked using scmd->eh_entry in this 38 discussion. 39 40 41 [1-2] How do scmd's get completed? 42 43 Once LLDD gets hold of a scmd, either the LLDD will complete the 44 command by calling scsi_done callback passed from midlayer when 45 invoking hostt->queuecommand() or SCSI midlayer will time it out. 46 47 48 [1-2-1] Completing a scmd w/ scsi_done 49 50 For all non-EH commands, scsi_done() is the completion callback. It 51 does the following. 52 53 1. Delete timeout timer. If it fails, it means that timeout timer 54 has expired and is going to finish the command. Just return. 55 56 2. Link scmd to per-cpu scsi_done_q using scmd->en_entry 57 58 3. Raise SCSI_SOFTIRQ 59 60 SCSI_SOFTIRQ handler scsi_softirq calls scsi_decide_disposition() to 61 determine what to do with the command. scsi_decide_disposition() 62 looks at the scmd->result value and sense data to determine what to do 63 with the command. 64 65 - SUCCESS 66 scsi_finish_command() is invoked for the command. The 67 function does some maintenance choirs and notify completion by 68 calling scmd->done() callback, which, for fs requests, would 69 be HLD completion callback - sd:sd_rw_intr, sr:rw_intr, 70 st:st_intr. 71 72 - NEEDS_RETRY 73 - ADD_TO_MLQUEUE 74 scmd is requeued to blk queue. 75 76 - otherwise 77 scsi_eh_scmd_add(scmd, 0) is invoked for the command. See 78 [1-3] for details of this function. 79 80 81 [1-2-2] Completing a scmd w/ timeout 82 83 The timeout handler is scsi_times_out(). When a timeout occurs, this 84 function 85 86 1. invokes optional hostt->eh_timed_out() callback. Return value can 87 be one of 88 89 - EH_HANDLED 90 This indicates that eh_timed_out() dealt with the timeout. The 91 scmd is passed to __scsi_done() and thus linked into per-cpu 92 scsi_done_q. Normal command completion described in [1-2-1] 93 follows. 94 95 - EH_RESET_TIMER 96 This indicates that more time is required to finish the 97 command. Timer is restarted. This action is counted as a 98 retry and only allowed scmd->allowed + 1(!) times. Once the 99 limit is reached, action for EH_NOT_HANDLED is taken instead. 100 101 *NOTE* This action is racy as the LLDD could finish the scmd 102 after the timeout has expired but before it's added back. In 103 such cases, scsi_done() would think that timeout has occurred 104 and return without doing anything. We lose completion and the 105 command will time out again. 106 107 - EH_NOT_HANDLED 108 This is the same as when eh_timed_out() callback doesn't exist. 109 Step #2 is taken. 110 111 2. scsi_eh_scmd_add(scmd, SCSI_EH_CANCEL_CMD) is invoked for the 112 command. See [1-3] for more information. 113 114 115 [1-3] How EH takes over 116 117 scmds enter EH via scsi_eh_scmd_add(), which does the following. 118 119 1. Turns on scmd->eh_eflags as requested. It's 0 for error 120 completions and SCSI_EH_CANCEL_CMD for timeouts. 121 122 2. Links scmd->eh_entry to shost->eh_cmd_q 123 124 3. Sets SHOST_RECOVERY bit in shost->shost_state 125 126 4. Increments shost->host_failed 127 128 5. Wakes up SCSI EH thread if shost->host_busy == shost->host_failed 129 130 As can be seen above, once any scmd is added to shost->eh_cmd_q, 131 SHOST_RECOVERY shost_state bit is turned on. This prevents any new 132 scmd to be issued from blk queue to the host; eventually, all scmds on 133 the host either complete normally, fail and get added to eh_cmd_q, or 134 time out and get added to shost->eh_cmd_q. 135 136 If all scmds either complete or fail, the number of in-flight scmds 137 becomes equal to the number of failed scmds - i.e. shost->host_busy == 138 shost->host_failed. This wakes up SCSI EH thread. So, once woken up, 139 SCSI EH thread can expect that all in-flight commands have failed and 140 are linked on shost->eh_cmd_q. 141 142 Note that this does not mean lower layers are quiescent. If a LLDD 143 completed a scmd with error status, the LLDD and lower layers are 144 assumed to forget about the scmd at that point. However, if a scmd 145 has timed out, unless hostt->eh_timed_out() made lower layers forget 146 about the scmd, which currently no LLDD does, the command is still 147 active as long as lower layers are concerned and completion could 148 occur at any time. Of course, all such completions are ignored as the 149 timer has already expired. 150 151 We'll talk about how SCSI EH takes actions to abort - make LLDD 152 forget about - timed out scmds later. 153 154 155  How SCSI EH works 156 157 LLDD's can implement SCSI EH actions in one of the following two 158 ways. 159 160 - Fine-grained EH callbacks 161 LLDD can implement fine-grained EH callbacks and let SCSI 162 midlayer drive error handling and call appropriate callbacks. 163 This will be discussed further in [2-1]. 164 165 - eh_strategy_handler() callback 166 This is one big callback which should perform whole error 167 handling. As such, it should do all choirs SCSI midlayer 168 performs during recovery. This will be discussed in [2-2]. 169 170 Once recovery is complete, SCSI EH resumes normal operation by 171 calling scsi_restart_operations(), which 172 173 1. Checks if door locking is needed and locks door. 174 175 2. Clears SHOST_RECOVERY shost_state bit 176 177 3. Wakes up waiters on shost->host_wait. This occurs if someone 178 calls scsi_block_when_processing_errors() on the host. 179 (*QUESTION* why is it needed? All operations will be blocked 180 anyway after it reaches blk queue.) 181 182 4. Kicks queues in all devices on the host in the asses 183 184 185 [2-1] EH through fine-grained callbacks 186 187 [2-1-1] Overview 188 189 If eh_strategy_handler() is not present, SCSI midlayer takes charge 190 of driving error handling. EH's goals are two - make LLDD, host and 191 device forget about timed out scmds and make them ready for new 192 commands. A scmd is said to be recovered if the scmd is forgotten by 193 lower layers and lower layers are ready to process or fail the scmd 194 again. 195 196 To achieve these goals, EH performs recovery actions with increasing 197 severity. Some actions are performed by issuing SCSI commands and 198 others are performed by invoking one of the following fine-grained 199 hostt EH callbacks. Callbacks may be omitted and omitted ones are 200 considered to fail always. 201 202 int (* eh_abort_handler)(struct scsi_cmnd *); 203 int (* eh_device_reset_handler)(struct scsi_cmnd *); 204 int (* eh_bus_reset_handler)(struct scsi_cmnd *); 205 int (* eh_host_reset_handler)(struct scsi_cmnd *); 206 207 Higher-severity actions are taken only when lower-severity actions 208 cannot recover some of failed scmds. Also, note that failure of the 209 highest-severity action means EH failure and results in offlining of 210 all unrecovered devices. 211 212 During recovery, the following rules are followed 213 214 - Recovery actions are performed on failed scmds on the to do list, 215 eh_work_q. If a recovery action succeeds for a scmd, recovered 216 scmds are removed from eh_work_q. 217 218 Note that single recovery action on a scmd can recover multiple 219 scmds. e.g. resetting a device recovers all failed scmds on the 220 device. 221 222 - Higher severity actions are taken iff eh_work_q is not empty after 223 lower severity actions are complete. 224 225 - EH reuses failed scmds to issue commands for recovery. For 226 timed-out scmds, SCSI EH ensures that LLDD forgets about a scmd 227 before reusing it for EH commands. 228 229 When a scmd is recovered, the scmd is moved from eh_work_q to EH 230 local eh_done_q using scsi_eh_finish_cmd(). After all scmds are 231 recovered (eh_work_q is empty), scsi_eh_flush_done_q() is invoked to 232 either retry or error-finish (notify upper layer of failure) recovered 233 scmds. 234 235 scmds are retried iff its sdev is still online (not offlined during 236 EH), REQ_FAILFAST is not set and ++scmd->retries is less than 237 scmd->allowed. 238 239 240 [2-1-2] Flow of scmds through EH 241 242 1. Error completion / time out 243 ACTION: scsi_eh_scmd_add() is invoked for scmd 244 - set scmd->eh_eflags 245 - add scmd to shost->eh_cmd_q 246 - set SHOST_RECOVERY 247 - shost->host_failed++ 248 LOCKING: shost->host_lock 249 250 2. EH starts 251 ACTION: move all scmds to EH's local eh_work_q. shost->eh_cmd_q 252 is cleared. 253 LOCKING: shost->host_lock (not strictly necessary, just for 254 consistency) 255 256 3. scmd recovered 257 ACTION: scsi_eh_finish_cmd() is invoked to EH-finish scmd 258 - shost->host_failed-- 259 - clear scmd->eh_eflags 260 - scsi_setup_cmd_retry() 261 - move from local eh_work_q to local eh_done_q 262 LOCKING: none 263 264 4. EH completes 265 ACTION: scsi_eh_flush_done_q() retries scmds or notifies upper 266 layer of failure. 267 - scmd is removed from eh_done_q and scmd->eh_entry is cleared 268 - if retry is necessary, scmd is requeued using 269 scsi_queue_insert() 270 - otherwise, scsi_finish_command() is invoked for scmd 271 LOCKING: queue or finish function performs appropriate locking 272 273 274 [2-1-3] Flow of control 275 276 EH through fine-grained callbacks start from scsi_unjam_host(). 277 278 <<scsi_unjam_host>> 279 280 1. Lock shost->host_lock, splice_init shost->eh_cmd_q into local 281 eh_work_q and unlock host_lock. Note that shost->eh_cmd_q is 282 cleared by this action. 283 284 2. Invoke scsi_eh_get_sense. 285 286 <<scsi_eh_get_sense>> 287 288 This action is taken for each error-completed 289 (!SCSI_EH_CANCEL_CMD) commands without valid sense data. Most 290 SCSI transports/LLDDs automatically acquire sense data on 291 command failures (autosense). Autosense is recommended for 292 performance reasons and as sense information could get out of 293 sync between occurrence of CHECK CONDITION and this action. 294 295 Note that if autosense is not supported, scmd->sense_buffer 296 contains invalid sense data when error-completing the scmd 297 with scsi_done(). scsi_decide_disposition() always returns 298 FAILED in such cases thus invoking SCSI EH. When the scmd 299 reaches here, sense data is acquired and 300 scsi_decide_disposition() is called again. 301 302 1. Invoke scsi_request_sense() which issues REQUEST_SENSE 303 command. If fails, no action. Note that taking no action 304 causes higher-severity recovery to be taken for the scmd. 305 306 2. Invoke scsi_decide_disposition() on the scmd 307 308 - SUCCESS 309 scmd->retries is set to scmd->allowed preventing 310 scsi_eh_flush_done_q() from retrying the scmd and 311 scsi_eh_finish_cmd() is invoked. 312 313 - NEEDS_RETRY 314 scsi_eh_finish_cmd() invoked 315 316 - otherwise 317 No action. 318 319 3. If !list_empty(&eh_work_q), invoke scsi_eh_abort_cmds(). 320 321 <<scsi_eh_abort_cmds>> 322 323 This action is taken for each timed out command. 324 hostt->eh_abort_handler() is invoked for each scmd. The 325 handler returns SUCCESS if it has succeeded to make LLDD and 326 all related hardware forget about the scmd. 327 328 If a timedout scmd is successfully aborted and the sdev is 329 either offline or ready, scsi_eh_finish_cmd() is invoked for 330 the scmd. Otherwise, the scmd is left in eh_work_q for 331 higher-severity actions. 332 333 Note that both offline and ready status mean that the sdev is 334 ready to process new scmds, where processing also implies 335 immediate failing; thus, if a sdev is in one of the two 336 states, no further recovery action is needed. 337 338 Device readiness is tested using scsi_eh_tur() which issues 339 TEST_UNIT_READY command. Note that the scmd must have been 340 aborted successfully before reusing it for TEST_UNIT_READY. 341 342 4. If !list_empty(&eh_work_q), invoke scsi_eh_ready_devs() 343 344 <<scsi_eh_ready_devs>> 345 346 This function takes four increasingly more severe measures to 347 make failed sdevs ready for new commands. 348 349 1. Invoke scsi_eh_stu() 350 351 <<scsi_eh_stu>> 352 353 For each sdev which has failed scmds with valid sense data 354 of which scsi_check_sense()'s verdict is FAILED, 355 START_STOP_UNIT command is issued w/ start=1. Note that 356 as we explicitly choose error-completed scmds, it is known 357 that lower layers have forgotten about the scmd and we can 358 reuse it for STU. 359 360 If STU succeeds and the sdev is either offline or ready, 361 all failed scmds on the sdev are EH-finished with 362 scsi_eh_finish_cmd(). 363 364 *NOTE* If hostt->eh_abort_handler() isn't implemented or 365 failed, we may still have timed out scmds at this point 366 and STU doesn't make lower layers forget about those 367 scmds. Yet, this function EH-finish all scmds on the sdev 368 if STU succeeds leaving lower layers in an inconsistent 369 state. It seems that STU action should be taken only when 370 a sdev has no timed out scmd. 371 372 2. If !list_empty(&eh_work_q), invoke scsi_eh_bus_device_reset(). 373 374 <<scsi_eh_bus_device_reset>> 375 376 This action is very similar to scsi_eh_stu() except that, 377 instead of issuing STU, hostt->eh_device_reset_handler() 378 is used. Also, as we're not issuing SCSI commands and 379 resetting clears all scmds on the sdev, there is no need 380 to choose error-completed scmds. 381 382 3. If !list_empty(&eh_work_q), invoke scsi_eh_bus_reset() 383 384 <<scsi_eh_bus_reset>> 385 386 hostt->eh_bus_reset_handler() is invoked for each channel 387 with failed scmds. If bus reset succeeds, all failed 388 scmds on all ready or offline sdevs on the channel are 389 EH-finished. 390 391 4. If !list_empty(&eh_work_q), invoke scsi_eh_host_reset() 392 393 <<scsi_eh_host_reset>> 394 395 This is the last resort. hostt->eh_host_reset_handler() 396 is invoked. If host reset succeeds, all failed scmds on 397 all ready or offline sdevs on the host are EH-finished. 398 399 5. If !list_empty(&eh_work_q), invoke scsi_eh_offline_sdevs() 400 401 <<scsi_eh_offline_sdevs>> 402 403 Take all sdevs which still have unrecovered scmds offline 404 and EH-finish the scmds. 405 406 5. Invoke scsi_eh_flush_done_q(). 407 408 <<scsi_eh_flush_done_q>> 409 410 At this point all scmds are recovered (or given up) and 411 put on eh_done_q by scsi_eh_finish_cmd(). This function 412 flushes eh_done_q by either retrying or notifying upper 413 layer of failure of the scmds. 414 415 416 [2-2] EH through transportt->eh_strategy_handler() 417 418 transportt->eh_strategy_handler() is invoked in the place of 419 scsi_unjam_host() and it is responsible for whole recovery process. 420 On completion, the handler should have made lower layers forget about 421 all failed scmds and either ready for new commands or offline. Also, 422 it should perform SCSI EH maintenance choirs to maintain integrity of 423 SCSI midlayer. IOW, of the steps described in [2-1-2], all steps 424 except for #1 must be implemented by eh_strategy_handler(). 425 426 427 [2-2-1] Pre transportt->eh_strategy_handler() SCSI midlayer conditions 428 429 The following conditions are true on entry to the handler. 430 431 - Each failed scmd's eh_flags field is set appropriately. 432 433 - Each failed scmd is linked on scmd->eh_cmd_q by scmd->eh_entry. 434 435 - SHOST_RECOVERY is set. 436 437 - shost->host_failed == shost->host_busy 438 439 440 [2-2-2] Post transportt->eh_strategy_handler() SCSI midlayer conditions 441 442 The following conditions must be true on exit from the handler. 443 444 - shost->host_failed is zero. 445 446 - Each scmd's eh_eflags field is cleared. 447 448 - Each scmd is in such a state that scsi_setup_cmd_retry() on the 449 scmd doesn't make any difference. 450 451 - shost->eh_cmd_q is cleared. 452 453 - Each scmd->eh_entry is cleared. 454 455 - Either scsi_queue_insert() or scsi_finish_command() is called on 456 each scmd. Note that the handler is free to use scmd->retries and 457 ->allowed to limit the number of retries. 458 459 460 [2-2-3] Things to consider 461 462 - Know that timed out scmds are still active on lower layers. Make 463 lower layers forget about them before doing anything else with 464 those scmds. 465 466 - For consistency, when accessing/modifying shost data structure, 467 grab shost->host_lock. 468 469 - On completion, each failed sdev must have forgotten about all 470 active scmds. 471 472 - On completion, each failed sdev must be ready for new commands or 473 offline. 474 475 476 -- 477 Tejun Heo 478 email@example.com 479 11th September 2005