Linux那些事儿之我是Block层(9)scsi命令的前世今生(三)

下一个更为重要的函数是scsi_dispatch_cmd,来自drivers/scsi/scsi.c:

459 /*
460 * Function: scsi_dispatch_command
461 *
462 * Purpose: Dispatch a command to the low-level driver.
463 *
464 * Arguments: cmd - command block we are dispatching.
465 *
466 * Notes:
467 */
468 int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
469 {
470 struct Scsi_Host *host = cmd->device->host;
471 unsigned long flags = 0;
472 unsigned long timeout;
473 int rtn = 0;
474
475 /* check if the device is still usable */
476 if (unlikely(cmd->device->sdev_state == SDEV_DEL)) {
477 /* in SDEV_DEL we error all commands. DID_NO_CONNECT
478 * returns an immediate error upwards, and signals
479 * that the device is no longer present */
480 cmd->result = DID_NO_CONNECT << 16;
481 atomic_inc(&cmd->device->iorequest_cnt);
482 __scsi_done(cmd);
483 /* return 0 (because the command has been processed) */
484 goto out;
485 }
486
487 /* Check to see if the scsi lld put this device into state SDEV_BLOCK. */
488 if (unlikely(cmd->device->sdev_state == SDEV_BLOCK)) {
489 /*
490 * in SDEV_BLOCK, the command is just put back on the device
491 * queue. The suspend state has already blocked the queue so
492 * future requests should not occur until the device
493 * transitions out of the suspend state.
494 */
495 scsi_queue_insert(cmd, SCSI_MLQUEUE_DEVICE_BUSY);
496
497 SCSI_LOG_MLQUEUE(3, printk("queuecommand : device blocked /n"));
498
499 /*
500 * NOTE: rtn is still zero here because we don't need the
501 * queue to be plugged on return (it's already stopped)
502 */
503 goto out;
504 }
505
506 /*
507 * If SCSI-2 or lower, store the LUN value in cmnd.
508 */
509 if (cmd->device->scsi_level <= SCSI_2 &&
510 cmd->device->scsi_level != SCSI_UNKNOWN) {
511 cmd->cmnd[1] = (cmd->cmnd[1] & 0x1f) |
512 (cmd->device->lun << 5 & 0xe0);
513 }
514
515 /*
516 * We will wait MIN_RESET_DELAY clock ticks after the last reset so
517 * we can avoid the drive not being ready.
518 */
519 timeout = host->last_reset + MIN_RESET_DELAY;
520
521 if (host->resetting && time_before(jiffies, timeout)) {
522 int ticks_remaining = timeout - jiffies;
523 /*
524 * NOTE: This may be executed from within an interrupt
525 * handler! This is bad, but for now, it'll do. The irq
526 * level of the interrupt handler has been masked out by the
527 * platform dependent interrupt handling code already, so the
528 * sti() here will not cause another call to the SCSI host's
529 * interrupt handler (assuming there is one irq-level per
530 * host).
531 */
532 while (--ticks_remaining >= 0)
533 mdelay(1 + 999 / HZ);
534 host->resetting = 0;
535 }
536
537 /*
538 * AK: unlikely race here: for some reason the timer could
539 * expire before the serial number is set up below.
540 */
541 scsi_add_timer(cmd, cmd->timeout_per_command, scsi_times_out);
542
543 scsi_log_send(cmd);
544
545 /*
546 * We will use a queued command if possible, otherwise we will
547 * emulate the queuing and calling of completion function ourselves.
548 */
549 atomic_inc(&cmd->device->iorequest_cnt);
550
551 /*
552 * Before we queue this command, check if the command
553 * length exceeds what the host adapter can handle.
554 */
555 if (CDB_SIZE(cmd) > cmd->device->host->max_cmd_len) {
556 SCSI_LOG_MLQUEUE(3,
557 printk("queuecommand : command too long./n"));
558 cmd->result = (DID_ABORT << 16);
559
560 scsi_done(cmd);
561 goto out;
562 }
563
564 spin_lock_irqsave(host->host_lock, flags);
565 scsi_cmd_get_serial(host, cmd);
566
567 if (unlikely(host->shost_state == SHOST_DEL)) {
568 cmd->result = (DID_NO_CONNECT << 16);
569 scsi_done(cmd);
570 } else {
571 rtn = host->hostt->queuecommand(cmd, scsi_done);
572 }
573 spin_unlock_irqrestore(host->host_lock, flags);
574 if (rtn) {
575 if (scsi_delete_timer(cmd)) {
576 atomic_inc(&cmd->device->iodone_cnt);
577 scsi_queue_insert(cmd,
578 (rtn == SCSI_MLQUEUE_DEVICE_BUSY) ?
579 rtn : SCSI_MLQUEUE_HOST_BUSY);
580 }
581 SCSI_LOG_MLQUEUE(3,
582 printk("queuecommand : request rejected/n"));
583 }
584
585 out:
586 SCSI_LOG_MLQUEUE(3, printk("leaving scsi_dispatch_cmnd()/n"));
587 return rtn;
588 }

一路走来的兄弟一定会一眼就看出这里我们最期待的一行代码就是571那个queuecommand()的调用.因为这之后我们就知道该发生什么了.比如对于U盘驱动来说,命令就从这里接过去开始执行.而对于实际的scsi控制器,其对应的驱动中的queuecommand也会被调用,剩下的事情我们就不用操心了.正常情况下queuecommand返回0.于是紧接着scsi_dispatch_cmd也返回0.这样就算是执行了一条scsi命令了.
而scsi_request_fn()是否结束还得看while循环的条件是否满足,而这就得看blk_queue_plugged()的脸色了.那么我们从字面上来分析,什么叫queue plugged?我那盗版金山词霸告诉我plugged就是塞紧的意思,你说队列塞紧的是什么意思?比如说,北四环上上下班高峰期,许许多多的车辆排成一队又一队,但是可能半天都前进不了,这就叫塞紧,或者说堵车,也叫塞车.为此咱们使用一个flag来标志堵车与否,来自include/linux/blkdev.h:

523 #define blk_queue_plugged(q) test_bit(QUEUE_FLAG_PLUGGED, &(q)->queue_flags)

改变这个这个flag的函数有两个,一个是设置,一个是取消.
负责设置的是blk_plug_device.

1542 /*
1543 * "plug" the device if there are no outstanding requests: this will
1544 * force the transfer to start only after we have put all the requests
1545 * on the list.
1546 *
1547 * This is called with interrupts off and no requests on the queue and
1548 * with the queue lock held.
1549 */
1550 void blk_plug_device(request_queue_t *q)
1551 {
1552 WARN_ON(!irqs_disabled());
1553
1554 /*
1555 * don't plug a stopped queue, it must be paired with blk_start_queue()
1556 * which will restart the queueing
1557 */
1558 if (blk_queue_stopped(q))
1559 return;
1560
1561 if (!test_and_set_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags)) {
1562 mod_timer(&q->unplug_timer, jiffies + q->unplug_delay);
1563 blk_add_trace_generic(q, NULL, 0, BLK_TA_PLUG);
1564 }
1565 }

负责取消的是blk_remove_plug().

1569 /*
1570 * remove the queue from the plugged list, if present. called with
1571 * queue lock held and interrupts disabled.
1572 */
1573 int blk_remove_plug(request_queue_t *q)
1574 {
1575 WARN_ON(!irqs_disabled());
1576
1577 if (!test_and_clear_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags))
1578 return 0;
1579
1580 del_timer(&q->unplug_timer);
1581 return 1;
1582 }

而调用前者的地方不少,比如我们见到的__elv_add_request,其第四个参数int plug就可以控制是否调用blk_plug_device(),而当我们在blk_execute_rq_nowait()中调用__elv_add_request()的时候传递的plug就是1.
另一方面,调用blk_remove_plug的地方也有多处.其中__generic_unplug_device()就是之一.所以在咱们这个上下文里,实际上并没有设置这个flag,因此scsi_request_fn()就会被执行.
那么编写这两个函数究竟是为了什么呢?这年头,有人做贼,我可以理解是为了劫富济贫,有人杀人,我可以理解是为了伸张正义,甚至有女人红杏出墙,我还可以理解是为了繁荣经济.然而,很长一段时间我都没办法理解有人编写这两个函数是为了什么?
后来我想,不妨这样理解,假设你经常开车经过长安街,你会发现经常有戒严的现象发生,比如某位领导人要出行,比如某位领导人要来访,而你可以把blk_plug_device()想象成戒严,把blk_remove_plug想象成开放.车流要想行进,前提条件是没有戒严,换言之,没有设卡,而QUEUE_FLAG_PLUGGED这个flag就相当于”卡”,设了它队列就不能前进了,没有设才有可能前进.之所以需要设卡,是因为确实有这个需求,有时候确实不想让队列前进.
那么这里我们还看到两个函数被调用了,mod_timer和del_timer,这是干嘛使的?还记得kblockd么?最早咱们创建了那个工作队列kblockd_workqueue,现在是它该出场的时间了.让我们把镜头拉回到函数blk_init_queue_node().这个函数我们曾经看过,所以这里只贴出其中跟我们这里密切相关的几行:

1922 q->request_fn = rfn;
1923 q->prep_rq_fn = NULL;
1924 q->unplug_fn = generic_unplug_device;
1925 q->queue_flags = (1 << QUEUE_FLAG_CLUSTER);
1926 q->queue_lock = lock;
1927
1928 blk_queue_segment_boundary(q, 0xffffffff);
1929
1930 blk_queue_make_request(q, __make_request);

首先q->unplug_fn被赋上了generic_unplug_device.这一点很重要,稍后会用到.
然后来看blk_queue_make_request().这个函数当时咱们并没有讲过.来自block/ll_rw_block.c:

180 /**
181 * blk_queue_make_request - define an alternate make_request function for a device
182 * @q: the request queue for the device to be affected
183 * @mfn: the alternate make_request function
184 *
185 * Description:
186 * The normal way for &struct bios to be passed to a device
187 * driver is for them to be collected into requests on a request
188 * queue, and then to allow the device driver to select requests
189 * off that queue when it is ready. This works well for many block
190 * devices. However some block devices (typically virtual devices
191 * such as md or lvm) do not benefit from the processing on the
192 * request queue, and are served best by having the requests passed
193 * directly to them. This can be achieved by providing a function
194 * to blk_queue_make_request().
195 *
196 * Caveat:
197 * The driver that does this *must* be able to deal appropriately
198 * with buffers in "highmemory". This can be accomplished by either calling
199 * __bio_kmap_atomic() to get a temporary kernel mapping, or by calling
200 * blk_queue_bounce() to create a buffer in normal memory.
201 **/
202 void blk_queue_make_request(request_queue_t * q, make_request_fn * mfn)
203 {
204 /*
205 * set defaults
206 */
207 q->nr_requests = BLKDEV_MAX_RQ;
208 blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);
209 blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
210 q->make_request_fn = mfn;
211 q->backing_dev_info.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
212 q->backing_dev_info.state = 0;
213 q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
214 blk_queue_max_sectors(q, SAFE_MAX_SECTORS);
215 blk_queue_hardsect_size(q, 512);
216 blk_queue_dma_alignment(q, 511);
217 blk_queue_congestion_threshold(q);
218 q->nr_batching = BLK_BATCH_REQ;
219
220 q->unplug_thresh = 4; /* hmm */
221 q->unplug_delay = (3 * HZ) / 1000; /* 3 milliseconds */
222 if (q->unplug_delay == 0)
223 q->unplug_delay = 1;
224
225 INIT_WORK(&q->unplug_work, blk_unplug_work);
226
227 q->unplug_timer.function = blk_unplug_timeout;
228 q->unplug_timer.data = (unsigned long)q;
229
230 /*
231 * by default assume old behaviour and bounce for any highmem page
232 */
233 blk_queue_bounce_limit(q, BLK_BOUNCE_HIGH);
234 }

这里重点关注几个”unplug”为名字的成员.尤其是INIT_WORK,它使得一旦unplug_work这项工作被执行,blk_unplug_work这个函数就会被执行.而unplug_timer这么一赋值,我们就知道,一旦设了闹钟,一旦闹钟时间到了,blk_unplug_timeout这个函数就会被执行.并且因为这里设置了unplug_delay为3ms,使得闹钟的timeout就是3ms,一旦激活闹钟,3ms之后blk_unplug_timeout就会被执行.这个函数来自block/ll_rw_blk.c:

1646 static void blk_unplug_timeout(unsigned long data)
1647 {
1648 request_queue_t *q = (request_queue_t *)data;
1649
1650 blk_add_trace_pdu_int(q, BLK_TA_UNPLUG_TIMER, NULL,
1651 q->rq.count[READ] + q->rq.count[WRITE]);
1652
1653 kblockd_schedule_work(&q->unplug_work);
1654 }

可以看到,其实就是执行kblockd_schedule_work,换言之,真正被调用的函数就是blk_unplug_work().

1636 static void blk_unplug_work(struct work_struct *work)
1637 {
1638 request_queue_t *q = container_of(work, request_queue_t, unplug_work);
1639
1640 blk_add_trace_pdu_int(q, BLK_TA_UNPLUG_IO, NULL,
1641 q->rq.count[READ] + q->rq.count[WRITE]);
1642
1643 q->unplug_fn(q);
1644 }

而刚才我们说了,unplug_fn被赋上了generic_unplug_device.所以真正要执行的是generic_unplug_device.而这个函数又长成什么样呢?

1601 /**
1602 * generic_unplug_device - fire a request queue
1603 * @q: The &request_queue_t in question
1604 *
1605 * Description:
1606 * Linux uses plugging to build bigger requests queues before letting
1607 * the device have at them. If a queue is plugged, the I/O scheduler
1608 * is still adding and merging requests on the queue. Once the queue
1609 * gets unplugged, the request_fn defined for the queue is invoked and
1610 * transfers started.
1611 **/
1612 void generic_unplug_device(request_queue_t *q)
1613 {
1614 spin_lock_irq(q->queue_lock);
1615 __generic_unplug_device(q);
1616 spin_unlock_irq(q->queue_lock);
1617 }

哦,扭扭捏捏大半天,其实就是调用__generic_unplug_device.而回过头去看这个函数,我们知道,它也无非就是调用了两个函数,blk_remove_plug和request_fn.这下子我们基本上就明白了.总结一下就是:
1. blk_plug_device()负责戒严.
2. blk_remove_plug()负责解禁.
3. 但是戒严这东西吧,也是有时间限制的,毕竟长安街就算有重大活动也是短时间的,一年中毕竟大多数时间还是得保证道路畅通.所以在戒严的时候,设了一个定时器,unplug_timer, (即mod_timer),一旦时间到了就自动执行blk_remove_plug去解禁.
4. 而在解禁的时候就不要忘记把这个定时器给关掉.(即del_timer)
5. 解禁之后调用request_fn()开始处理队列中的下一个请求,或者说车流开始恢复前行.
Ok,这样我们就算是明白这两个戒严与解禁的函数了.最后,题外话,关于unplug和plug,我觉得更贴切的单词是activate和deactivate,或者说激活与冻结,或者简单的说,开与关.