上一節講到在raid5的同步函數sync_request中炸土豆片是通過handle_stripe來進行的。從最初的創建陣列，到申請各種資源，建立每個陣列的personality，所有的一切都是為了迎接數據流而作的准備。就像我們寒窗苦讀就是為了上大學一樣。數據流的過程就像大學校園一樣豐富多彩並且富有挑戰性，但只要跨過了這道坎，內核代碼將不再神秘，剩下的問題只是時間而已。
首先看handle_stripe究竟把我們的土豆片帶往何處：
3379 static void handle_stripe(struct stripe_head *sh)
3380 {
3381 struct stripe_head_state s;
3382 struct r5conf *conf = sh->raid_conf;
3383 int i;
3384 int prexor;
3385 int disks = sh->disks;
3386 struct r5dev *pdev, *qdev;
3387
3388 clear_bit(STRIPE_HANDLE, &sh->state);
3389 if (test_and_set_bit_lock(STRIPE_ACTIVE, &sh->state)) {
3390 /* already being handled, ensure it gets handled
3391 * again when current action finishes */
3392 set_bit(STRIPE_HANDLE, &sh->state);
3393 return;
3394 }
3395
3396 if (test_and_clear_bit(STRIPE_SYNC_REQUESTED, &sh->state)) {
3397 set_bit(STRIPE_SYNCING, &sh->state);
3398 clear_bit(STRIPE_INSYNC, &sh->state);
3399 }
3400 clear_bit(STRIPE_DELAYED, &sh->state);
3401
3402 pr_debug("handling stripe %llu, state=%#lx cnt=%d, "
3403 "pd_idx=%d, qd_idx=%d\n, check:%d, reconstruct:%d\n",
3404 (unsigned long long)sh->sector, sh->state,
3405 atomic_read(&sh->count), sh->pd_idx, sh->qd_idx,
3406 sh->check_state, sh->reconstruct_state);
3407
3408 analyse_stripe(sh, &s);
這個函數代碼比較長先貼第一部分，分析條帶。分析的作用就是根據條帶的狀態做一些預處理，根據這些狀態再來判斷下一步應該做什麼具體操作。比如說同步，那麼首先會讀數據盤，等讀回來之後，再校驗，然後再寫校驗值。但是這些步驟又不是一次性在handle_stripe裡就完成的，因為跟磁盤IO都是異步的，所以必要要等上一次磁盤請求回調之後再次調用handle_stripe，通常每個數據流都會多次進入handle_stripe，而每一次進入經過的代碼流程是不大一樣的。
struct stripe_head有很多狀態，這些狀態決定條帶應該怎麼處理，所以必須非常小心處理這些標志，這些標志很多，現在先簡單地過一下。
enum {
STRIPE_ACTIVE, // 正在處理
STRIPE_HANDLE, // 需要處理
STRIPE_SYNC_REQUESTED, // 同步請求
STRIPE_SYNCING, // 正在處理同步
STRIPE_INSYNC, // 條帶已同步
STRIPE_PREREAD_ACTIVE, // 預讀
STRIPE_DELAYED, // 延遲處理
STRIPE_DEGRADED, // 降級
STRIPE_BIT_DELAY, // 等待bitmap處理
STRIPE_EXPANDING, //
STRIPE_EXPAND_SOURCE, //
STRIPE_EXPAND_READY, //
STRIPE_IO_STARTED, /* do not count towards 'bypass_count' */ // IO已下發
STRIPE_FULL_WRITE, /* all blocks are set to be overwritten */ // 滿寫
STRIPE_BIOFILL_RUN, // bio填充，就是將page頁拷貝到bio
STRIPE_COMPUTE_RUN, // 運行計算
STRIPE_OPS_REQ_PENDING, // handle_stripe排隊用
STRIPE_ON_UNPLUG_LIST, // 批量release_stripe時標識是否加入unplug鏈表
};
3388行，清除需要處理標志。
3389行，設置正在處理標志。
3392行，如果已經在處理則設置下次處理標志並返回。
3396行，如果是同步請求。
3397行，設置正在處理同步標志。
3398行，清除已同步標志。
3400行，清除延遲處理標志。
3408行，分析stripe，這個函數很長分幾段來說明：
3198 static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
3199 {
3200 struct r5conf *conf = sh->raid_conf;
3201 int disks = sh->disks;
3202 struct r5dev *dev;
3203 int i;
3204 int do_recovery = 0;
3205
3206 memset(s, 0, sizeof(*s));
3207
3208 s->expanding = test_bit(STRIPE_EXPAND_SOURCE, &sh->state);
3209 s->expanded = test_bit(STRIPE_EXPAND_READY, &sh->state);
3210 s->failed_num[0] = -1;
3211 s->failed_num[1] = -1;
3212
3213 /* Now to look around and see what can be done */
3214 rcu_read_lock();
數據初始化和加鎖，接著看：
3215 for (i=disks; i--; ) {
3216 struct md_rdev *rdev;
3217 sector_t first_bad;
3218 int bad_sectors;
3219 int is_bad = 0;
3220
3221 dev = &sh->dev[i];
3222
3223 pr_debug("check %d: state 0x%lx read %p write %p written %p\n",
3224 i, dev->flags,
3225 dev->toread, dev->towrite, dev->written);
接著是一個大循環，循環次數是數據盤的個數，循環的對象的3221行的dev，dev的類型是struct r5dev，那我們先來看一看這個結構，這個結構是嵌套在struct stripe_head裡面的：
struct r5dev {
/* rreq and rvec are used for the replacement device when
* writing data to both devices.
*/
struct bio req, rreq;
struct bio_vec vec, rvec;
struct page *page;
struct bio *toread, *read, *towrite, *written;
sector_t sector; /* sector of this page */
unsigned long flags;
} dev[1]; /* allocated with extra space depending of RAID geometry */
首先看注釋，rreq 和rvec由replacement設備在寫數據時使用。r就是replacement的簡寫，replacement是什麼意思呢？就是原數據盤的替代，replacement是最近幾個版本裡才引入的特性，在實際產品中這個特性很重要，具體實現後面會講到。page是緩存頁，通常用於運行計算，接著幾個bio是讀寫bio頭指針。sector是條帶對應的物理扇區位置。flags是struct r5dev的標志。
3226 /* maybe we can reply to a read
3227 *
3228 * new wantfill requests are only permitted while
3229 * ops_complete_biofill is guaranteed to be inactive
3230 */
3231 if (test_bit(R5_UPTODATE, &dev->flags) && dev->toread &&
3232 !test_bit(STRIPE_BIOFILL_RUN, &sh->state))
3233 set_bit(R5_Wantfill, &dev->flags);
3234
3235 /* now count some things */
3236 if (test_bit(R5_LOCKED, &dev->flags))
3237 s->locked++;
3238 if (test_bit(R5_UPTODATE, &dev->flags))
3239 s->uptodate++;
3240 if (test_bit(R5_Wantcompute, &dev->flags)) {
3241 s->compute++;
3242 BUG_ON(s->compute > 2);
3243 }
3244
3245 if (test_bit(R5_Wantfill, &dev->flags))
3246 s->to_fill++;
3247 else if (dev->toread)
3248 s->to_read++;
3249 if (dev->towrite) {
3250 s->to_write++;
3251 if (!test_bit(R5_OVERWRITE, &dev->flags))
3252 s->non_overwrite++;
3253 }
3254 if (dev->written)
3255 s->written++;
3231行，什麼樣的r5dev要設置R5_Wantfill標志呢？已更新、有讀請求、不在拷貝過程。這又是什麼意思呢？就是說需要的數據已經為最新了，這時只要把數據從page拷貝到bio就可以了。
3236行，統計加鎖磁盤數
3238行，統計已最新磁盤數
3240行，統計需要計算的磁盤數
3245行，統計需要拷貝操作磁盤數
3247行，統計需要讀的磁盤數
3249行，統計需要寫的磁盤數
3251行，統計滿寫的磁盤數
3254行，統計已下發寫的磁盤數
3256 /* Prefer to use the replacement for reads, but only
3257 * if it is recovered enough and has no bad blocks.
3258 */
3259 rdev = rcu_dereference(conf->disks[i].replacement);
3260 if (rdev && !test_bit(Faulty, &rdev->flags) &&
3261 rdev->recovery_offset >= sh->sector + STRIPE_SECTORS &&
3262 !is_badblock(rdev, sh->sector, STRIPE_SECTORS,
3263 &first_bad, &bad_sectors))
3264 set_bit(R5_ReadRepl, &dev->flags);
3265 else {
3266 if (rdev)
3267 set_bit(R5_NeedReplace, &dev->flags);
3268 rdev = rcu_dereference(conf->disks[i].rdev);
3269 clear_bit(R5_ReadRepl, &dev->flags);
3270 }
3271 if (rdev && test_bit(Faulty, &rdev->flags))
3272 rdev = NULL;
3273 if (rdev) {
3274 is_bad = is_badblock(rdev, sh->sector, STRIPE_SECTORS,
3275 &first_bad, &bad_sectors);
3276 if (s->blocked_rdev == NULL
3277 && (test_bit(Blocked, &rdev->flags)
3278 || is_bad < 0)) {
3279 if (is_bad < 0)
3280 set_bit(BlockedBadBlocks,
3281 &rdev->flags);
3282 s->blocked_rdev = rdev;
3283 atomic_inc(&rdev->nr_pending);
3284 }
3285 }
3256行，優先讀重建過並沒有壞扇區的replacement盤
3264行，讀replacement盤
3267行，寫replacement盤
3271行，壞盤
3273行，檢查壞扇區
3286行，初始化dev狀態
3300行，沒有壞扇區，設置同步標志
3312行，寫錯誤處理
3325行，數據盤修復處理
3336行，replacement盤修復處理
3352行，記錄不同步盤
3360行，判斷同步還是重構replacement盤
到此analyse_stripe就結束了，那麼對於同步來說，這個函數做了哪些事情呢？就只是設置了s.syncing=1而已，所以不要看這個函數那麼長，每一次進來做的事情卻很少。
繼續返回到handle_stripe函數中，中間不執行的代碼先跳過，然後就會執行到這裡：
3468 /* Now we might consider reading some blocks, either to check/generate 3469 * parity, or to satisfy requests 3470 * or to load a block that is being partially written. 3471 */ 3472 if (s.to_read || s.non_overwrite 3473 || (conf->level == 6 && s.to_write && s.failed) 3474 || (s.syncing && (s.uptodate + s.compute < disks)) 3475 || s.replacing 3476 || s.expanding) 3477 handle_stripe_fill(sh, &s, disks);
3468行，這裡是准備讀磁盤，在生成校驗、讀寫請求時都有可能讀磁盤
3474行，在analyse_stripe中設置了syncing標志，所以這裡滿足這個條件，進入handle_stripe_fill函數。
2707 /**
2708 * handle_stripe_fill - read or compute data to satisfy pending requests.
2709 */
2710 static void handle_stripe_fill(struct stripe_head *sh,
2711 struct stripe_head_state *s,
2712 int disks)
2713 {
2714 int i;
2715
2716 /* look for blocks to read/compute, skip this if a compute
2717 * is already in flight, or if the stripe contents are in the
2718 * midst of changing due to a write
2719 */
2720 if (!test_bit(STRIPE_COMPUTE_RUN, &sh->state) && !sh->check_state &&
2721 !sh->reconstruct_state)
2722 for (i = disks; i--; )
2723 if (fetch_block(sh, s, i, disks))
2724 break;
2725 set_bit(STRIPE_HANDLE, &sh->state);
2726 }
2720行，如果已經在計算、校驗或重建狀態，則不需要再讀磁盤
2722行，循環每一個r5dev看是否需要讀磁盤
跟進fetch_block函數：
2618 /* fetch_block - checks the given member device to see if its data needs
2619 * to be read or computed to satisfy a request.
2620 *
2621 * Returns 1 when no more member devices need to be checked, otherwise returns
2622 * 0 to tell the loop in handle_stripe_fill to continue
2623 */
查看指定設置是否有必要讀入數據，返回1表示剩余設置不需要檢查了，返回0表示需要繼續檢查剩余的設置。
2624 static int fetch_block(struct stripe_head *sh, struct stripe_head_state *s,
2625 int disk_idx, int disks)
2626 {
2627 struct r5dev *dev = &sh->dev[disk_idx];
2628 struct r5dev *fdev[2] = { &sh->dev[s->failed_num[0]],
2629 &sh->dev[s->failed_num[1]] };
2630
2631 /* is the data in this block needed, and can we get it? */
2632 if (!test_bit(R5_LOCKED, &dev->flags) &&
2633 !test_bit(R5_UPTODATE, &dev->flags) &&
2634 (dev->toread ||
2635 (dev->towrite && !test_bit(R5_OVERWRITE, &dev->flags)) ||
2636 s->syncing || s->expanding ||
2637 (s->replacing && want_replace(sh, disk_idx)) ||
2638 (s->failed >= 1 && fdev[0]->toread) ||
2639 (s->failed >= 2 && fdev[1]->toread) ||
2640 (sh->raid_conf->level <= 5 && s->failed && fdev[0]->towrite &&
2641 !test_bit(R5_OVERWRITE, &fdev[0]->flags)) ||
2642 (sh->raid_conf->level == 6 && s->failed && s->to_write))) {
2643 /* we would like to get this block, possibly by computing it,
2644 * otherwise read it if the backing disk is insync
2645 */
2646 BUG_ON(test_bit(R5_Wantcompute, &dev->flags));
2647 BUG_ON(test_bit(R5_Wantread, &dev->flags));
2648 if ((s->uptodate == disks - 1) &&
2649 (s->failed && (disk_idx == s->failed_num[0] ||
2650 disk_idx == s->failed_num[1]))) {
2651 /* have disk failed, and we're requested to fetch it;
2652 * do compute it
2653 */
2654 pr_debug("Computing stripe %llu block %d\n",
2655 (unsigned long long)sh->sector, disk_idx);
2656 set_bit(STRIPE_COMPUTE_RUN, &sh->state);
2657 set_bit(STRIPE_OP_COMPUTE_BLK, &s->ops_request);
2658 set_bit(R5_Wantcompute, &dev->flags);
2659 sh->ops.target = disk_idx;
2660 sh->ops.target2 = -1; /* no 2nd target */
2661 s->req_compute = 1;
2662 /* Careful: from this point on 'uptodate' is in the eye
2663 * of raid_run_ops which services 'compute' operations
2664 * before writes. R5_Wantcompute flags a block that will
2665 * be R5_UPTODATE by the time it is needed for a
2666 * subsequent operation.
2667 */
2668 s->uptodate++;
2669 return 1;
2670 } else if (s->uptodate == disks-2 && s->failed >= 2) {
2671 /* Computing 2-failure is *very* expensive; only
2672 * do it if failed >= 2
2673 */
2674 int other;
2675 for (other = disks; other--; ) {
2676 if (other == disk_idx)
2677 continue;
2678 if (!test_bit(R5_UPTODATE,
2679 &sh->dev[other].flags))
2680 break;
2681 }
2682 BUG_ON(other < 0);
2683 pr_debug("Computing stripe %llu blocks %d,%d\n",
2684 (unsigned long long)sh->sector,
2685 disk_idx, other);
2686 set_bit(STRIPE_COMPUTE_RUN, &sh->state);
2687 set_bit(STRIPE_OP_COMPUTE_BLK, &s->ops_request);
2688 set_bit(R5_Wantcompute, &sh->dev[disk_idx].flags);
2689 set_bit(R5_Wantcompute, &sh->dev[other].flags);
2690 sh->ops.target = disk_idx;
2691 sh->ops.target2 = other;
2692 s->uptodate += 2;
2693 s->req_compute = 1;
2694 return 1;
2695 } else if (test_bit(R5_Insync, &dev->flags)) {
2696 set_bit(R5_LOCKED, &dev->flags);
2697 set_bit(R5_Wantread, &dev->flags);
2698 s->locked++;
2699 pr_debug("Reading block %d (sync=%d)\n",
2700 disk_idx, s->syncing);
2701 }
2702 }
2703
2704 return 0;
2705 }
從這個函數進入，我們擁有的僅僅是s.syncing這張牌，那麼這張牌在這裡能不能發揮作用呢？
2632行，判斷是否需要讀設置
2636行，很明顯地，這個判斷為真，因為s.syncing==1，其他判斷暫且不看
2648行，當前設置都未讀入，所以s->uptodate==0
2670行，同上也不成立
2695行，真正執行到的是這個分支
2696行，設置設備加鎖標志
2697行，設置設備准備讀標志
2698行，遞增本條帶加鎖設備數
handle_stripe函數執行完成，條帶的每個struct r5dev都被設置了R5_Wantread標志。在接下來handle_stripe就會調用ops_run_io函數去讀：
3673 ops_run_io(sh, &s);
我們再跟進這個函數，為了突出重點，這裡只列出跟同步相關的代碼：
537 static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
538 {
539 struct r5conf *conf = sh->raid_conf;
540 int i, disks = sh->disks;
541
542 might_sleep();
543
544 for (i = disks; i--; ) {
545 int rw;
546 int replace_only = 0;
547 struct bio *bi, *rbi;
548 struct md_rdev *rdev, *rrdev = NULL;
...
554 } else if (test_and_clear_bit(R5_Wantread, &sh->dev[i].flags))
555 rw = READ;
...
560 } else
561 continue;
564
565 bi = &sh->dev[i].req;
566 rbi = &sh->dev[i].rreq; /* For writing to replacement */
567
568 bi->bi_rw = rw;
569 rbi->bi_rw = rw;
570 if (rw & WRITE) {
573 } else
574 bi->bi_end_io = raid5_end_read_request;
575
576 rcu_read_lock();
577 rrdev = rcu_dereference(conf->disks[i].replacement);
578 smp_mb(); /* Ensure that if rrdev is NULL, rdev won't be */
579 rdev = rcu_dereference(conf->disks[i].rdev);
580 if (!rdev) {
581 rdev = rrdev;
582 rrdev = NULL;
583 }
...
598 if (rdev)
599 atomic_inc(&rdev->nr_pending);
...
604 rcu_read_unlock();
...
643 if (rdev) {
644 if (s->syncing || s->expanding || s->expanded
645 || s->replacing)
646 md_sync_acct(rdev->bdev, STRIPE_SECTORS);
647
648 set_bit(STRIPE_IO_STARTED, &sh->state);
649
650 bi->bi_bdev = rdev->bdev;
651 pr_debug("%s: for %llu schedule op %ld on disc %d\n",
652 __func__, (unsigned long long)sh->sector,
653 bi->bi_rw, i);
654 atomic_inc(&sh->count);
655 if (use_new_offset(conf, sh))
656 bi->bi_sector = (sh->sector
657 + rdev->new_data_offset);
658 else
659 bi->bi_sector = (sh->sector
660 + rdev->data_offset);
661 if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
662 bi->bi_rw |= REQ_FLUSH;
663
664 bi->bi_flags = 1 << BIO_UPTODATE;
665 bi->bi_idx = 0;
666 bi->bi_io_vec[0].bv_len = STRIPE_SIZE;
667 bi->bi_io_vec[0].bv_offset = 0;
668 bi->bi_size = STRIPE_SIZE;
669 bi->bi_next = NULL;
670 if (rrdev)
671 set_bit(R5_DOUBLE_LOCKED, &sh->dev[i].flags);
672 generic_make_request(bi);
673 }
...
709 }
710 }
542行，函數可能休眠
544行，遍歷每一個r5dev
554行，設置讀標志
568行，設置bio為讀
574行，設置bio回調函數為raid5_end_read_request，這裡將是下發讀請求之後代碼繼續執行的入口點。
598行，增加設備nr_pending
646行，統計信息
648行，設置IO下發標志
650行，設置bio設備為對應的磁盤設備
654行，增加stripe_head引用計數
655-660行，設置新的扇區數，需要加上磁盤上的數據偏移
661行，如果為NoMerge讀，則設置bio REQ_FLUSH標志
664行，接著設置bio其他域
672行，下發bio到磁盤
在磁盤執行完讀請求的時候，raid5_end_read_request被調用：
1710 static void raid5_end_read_request(struct bio * bi, int error)
1711 {
...
1824 rdev_dec_pending(rdev, conf->mddev);
1825 clear_bit(R5_LOCKED, &sh->dev[i].flags);
1826 set_bit(STRIPE_HANDLE, &sh->state);
1827 release_stripe(sh);
1828 }
在這個函數中，清除了R5_LOCKED標志，並重新將stripe_head加入處理。經過raid5d中轉，重新調用到handle_stripe函數，這一次調用時在analyse_stripe函數中遞增s->uptodate，所有數據盤都遞增1，所以s->uptodate等於數據盤。接著handle_tripe函數到達：
3528 if (sh->check_state ||
3529 (s.syncing && s.locked == 0 &&
3530 !test_bit(STRIPE_COMPUTE_RUN, &sh->state) &&
3531 !test_bit(STRIPE_INSYNC, &sh->state))) {
3532 if (conf->level == 6)
3533 handle_parity_checks6(conf, sh, &s, disks);
3534 else
3535 handle_parity_checks5(conf, sh, &s, disks);
3536 }
進入3535行進行校驗，進入handle_parity_check5函數：
2881 switch (sh->check_state) {
2882 case check_state_idle:
2883 /* start a new check operation if there are no failures */
2884 if (s->failed == 0) {
2885 BUG_ON(s->uptodate != disks);
2886 sh->check_state = check_state_run;
2887 set_bit(STRIPE_OP_CHECK, &s->ops_request);
2888 clear_bit(R5_UPTODATE, &sh->dev[sh->pd_idx].flags);
2889 s->uptodate--;
2890 break;
2891 }
2881行，check_state為0，進入2882行分支
2886行，設置check_state_run狀態
2887行，設置STRIPE_OP_CHECK操作
2889行，遞減s->uptodate
由於這裡設置了STRIPE_OP_CHECK操作，所以在handle_stripe會調用到raid_run_ops，進而會調用到：
1412 if (test_bit(STRIPE_OP_CHECK, &ops_request)) {
1413 if (sh->check_state == check_state_run)
1414 ops_run_check_p(sh, percpu);
ops_run_check_p校驗條帶是否同步，對應的回調函數為：
1301static void ops_complete_check(void *stripe_head_ref)
1302{
1303 struct stripe_head *sh = stripe_head_ref;
1304
1305 pr_debug("%s: stripe %llu\n", __func__,
1306 (unsigned long long)sh->sector);
1307
1308 sh->check_state = check_state_check_result;
1309 set_bit(STRIPE_HANDLE, &sh->state);
1310 release_stripe(sh);
1311}
第1308行將狀態設置為check_state_check_result，條帶繼續又重新加入到handle_list。handle_stripe再一次調用到handle_parity_check5函數，但這一次check_state==check_state_check_result：
2916 case check_state_check_result:
2917 sh->check_state = check_state_idle;
2918
2919 /* if a failure occurred during the check operation, leave
2920 * STRIPE_INSYNC not set and let the stripe be handled again
2921 */
2922 if (s->failed)
2923 break;
2924
2925 /* handle a successful check operation, if parity is correct
2926 * we are done. Otherwise update the mismatch count and repair
2927 * parity if !MD_RECOVERY_CHECK
2928 */
2929 if ((sh->ops.zero_sum_result & SUM_CHECK_P_RESULT) == 0)
2930 /* parity is correct (on disc,
2931 * not in buffer any more)
2932 */
2933 set_bit(STRIPE_INSYNC, &sh->state);
2934 else {
2935 conf->mddev->resync_mismatches += STRIPE_SECTORS;
2936 if (test_bit(MD_RECOVERY_CHECK, &conf->mddev->recovery))
2937 /* don't try to repair!! */
2938 set_bit(STRIPE_INSYNC, &sh->state);
2939 else {
2940 sh->check_state = check_state_compute_run;
2941 set_bit(STRIPE_COMPUTE_RUN, &sh->state);
2942 set_bit(STRIPE_OP_COMPUTE_BLK, &s->ops_request);
2943 set_bit(R5_Wantcompute,
2944 &sh->dev[sh->pd_idx].flags);
2945 sh->ops.target = sh->pd_idx;
2946 sh->ops.target2 = -1;
2947 s->uptodate++;
2948 }
2949 }
2950 break;
2929行，如果校驗的結果是同步的
2933行，直接設置條帶為同步的，不需要進行其他任何操作了
2934行，如果條帶不同步
2940行，設置check_state為check_state_compute_run
2942行，ops_request 為STRIPE_OP_COMPUTE_BLK，即准備計算校驗
2943行，計算目標為條帶校驗盤
2947行，由於之前計算校驗時uptodate遞減，這裡恢復
如果條帶已經同步了，那麼帶著STRIPE_INSYNC標志我們來到了handle_stripe：
3550 if ((s.syncing || s.replacing) && s.locked == 0 &&
3551 test_bit(STRIPE_INSYNC, &sh->state)) {
3552 md_done_sync(conf->mddev, STRIPE_SECTORS, 1);
3553 clear_bit(STRIPE_SYNCING, &sh->state);
3554 }
如果條帶未同步，那帶著STRIPE_OP_COMPUTE_BLK標志來到了raid_run_ops函數，該函數調用__raid_run_ops：
1383 if (test_bit(STRIPE_OP_COMPUTE_BLK, &ops_request)) {
1384 if (level < 6)
1385 tx = ops_run_compute5(sh, percpu);
最終調用ops_run_compute5函數計算出條帶中校驗盤的值，該函數回調函數ops_complete_compute：
856static void ops_complete_compute(void *stripe_head_ref)
857{
858 struct stripe_head *sh = stripe_head_ref;
859
860 pr_debug("%s: stripe %llu\n", __func__,
861 (unsigned long long)sh->sector);
862
863 /* mark the computed target(s) as uptodate */
864 mark_target_uptodate(sh, sh->ops.target);
865 mark_target_uptodate(sh, sh->ops.target2);
866
867 clear_bit(STRIPE_COMPUTE_RUN, &sh->state);
868 if (sh->check_state == check_state_compute_run)
869 sh->check_state = check_state_compute_result;
870 set_bit(STRIPE_HANDLE, &sh->state);
871 release_stripe(sh);
872}
864行，設置校驗盤dev為R5_UPTODATE
869行，由於handle_parity_check5中設置為check_state_compute_run，這裡繼續設置為check_state_compute_result
870行，設置處理標志，在871之後再一次進入handle_stripe
當再一次進入handle_stripe函數，又再一次來到handle_parity_check5函數，由於這次是check_state_compute_result標志：
2894 case check_state_compute_result: 2895 sh->check_state = check_state_idle; 2896 if (!dev) 2897 dev = &sh->dev[sh->pd_idx]; 2898 2899 /* check that a write has not made the stripe insync */ 2900 if (test_bit(STRIPE_INSYNC, &sh->state)) 2901 break; 2902 2903 /* either failed parity check, or recovery is happening */ 2904 BUG_ON(!test_bit(R5_UPTODATE, &dev->flags)); 2905 BUG_ON(s->uptodate != disks); 2906 2907 set_bit(R5_LOCKED, &dev->flags); 2908 s->locked++; 2909 set_bit(R5_Wantwrite, &dev->flags); 2910 2911 clear_bit(STRIPE_DEGRADED, &sh->state); 2912 set_bit(STRIPE_INSYNC, &sh->state); 2913 break;
我們可以一眼看2912行設置了STRIPE_INSYNC標志，那麼也意味著條帶同步的結束。但是也別高興得太早，回頭看卻有2908行s->locked++，同步結束的判斷條件之一就是s->locked==0，所以在同步結束之前我們還有一件事情要做，2909行設置了R5_Wantwrite標志就是告訴我們需要調用一次ops_run_io將剛才計算的校驗值寫入條帶的校驗盤中，再寫成功再返回時就會滿足同步結束的條件了。就這樣，一次簡單的同步過程就完成了。
出處：http://blog.csdn.net/liumangxiong