RAID5 wasn't designed to support overlapping requests because
in Linux all I/Os are serialized by page/buffer lock.  As Lustre
doesn't use pagecache on server, we need to serialize I/Os in RAID5.

Index: linux-2.6.9/include/linux/raid/raid5.h
===================================================================
--- linux-2.6.9.orig/include/linux/raid/raid5.h	2006-05-22 00:11:21.000000000 +0400
+++ linux-2.6.9/include/linux/raid/raid5.h	2006-05-22 00:11:21.000000000 +0400
@@ -134,6 +134,7 @@ struct stripe_head {
 	unsigned long		state;			/* state flags */
 	atomic_t		count;			/* nr of active thread/requests */
 	spinlock_t		lock;
+	wait_queue_head_t	wait;			/* waitchan for overlapped bio's */
 	struct r5dev {
 		struct bio	req;
 		struct bio_vec	vec;
Index: linux-2.6.9/drivers/md/raid5.c
===================================================================
--- linux-2.6.9.orig/drivers/md/raid5.c	2006-05-22 00:11:21.000000000 +0400
+++ linux-2.6.9/drivers/md/raid5.c	2006-05-22 00:19:27.000000000 +0400
@@ -308,6 +308,7 @@ static int grow_stripes(raid5_conf_t *co
 		memset(sh, 0, sizeof(*sh) + (devs-1)*sizeof(struct r5dev));
 		sh->raid_conf = conf;
 		sh->lock = SPIN_LOCK_UNLOCKED;
+		init_waitqueue_head(&sh->wait);
 
 		if (grow_buffers(sh, conf->raid_disks)) {
 			shrink_buffers(sh, conf->raid_disks);
@@ -878,6 +879,9 @@ static void compute_parity(struct stripe
 		set_bit(R5_LOCKED,   &sh->dev[pd_idx].flags);
 	} else
 		clear_bit(R5_UPTODATE, &sh->dev[pd_idx].flags);
+
+	/* probably someone waits for our completion? */
+	wake_up(&sh->wait);
 }
 
 /*
@@ -885,7 +889,7 @@ static void compute_parity(struct stripe
  * toread/towrite point to the first in a chain. 
  * The bi_next chain must be in order.
  */
-static void add_stripe_bio (struct stripe_head *sh, struct bio *bi, int dd_idx, int forwrite)
+static int add_stripe_bio (struct stripe_head *sh, struct bio *bi, int dd_idx, int forwrite)
 {
 	struct bio **bip;
 	raid5_conf_t *conf = sh->raid_conf;
@@ -894,13 +898,21 @@ static void add_stripe_bio (struct strip
 		(unsigned long long)bi->bi_sector,
 		(unsigned long long)sh->sector);
 
-
 	spin_lock(&sh->lock);
 	spin_lock_irq(&conf->device_lock);
 	if (forwrite)
 		bip = &sh->dev[dd_idx].towrite;
 	else
 		bip = &sh->dev[dd_idx].toread;
+
+#if 1
+	if (*bip) {
+		/* overlapping bio, let's wait till first one is completed */
+		spin_unlock_irq(&conf->device_lock);
+		spin_unlock(&sh->lock);
+		return 1;
+	}
+#else
 	while (*bip && (*bip)->bi_sector < bi->bi_sector) {
 		BUG_ON((*bip)->bi_sector + ((*bip)->bi_size >> 9) > bi->bi_sector);
 		bip = & (*bip)->bi_next;
@@ -910,6 +922,7 @@ static void add_stripe_bio (struct strip
 		BUG();
 	if (*bip)
 		bi->bi_next = *bip;
+#endif
 	*bip = bi;
 	bi->bi_phys_segments ++;
 	spin_unlock_irq(&conf->device_lock);
@@ -932,6 +945,7 @@ static void add_stripe_bio (struct strip
 		if (sector >= sh->dev[dd_idx].sector + STRIPE_SECTORS)
 			set_bit(R5_OVERWRITE, &sh->dev[dd_idx].flags);
 	}
+	return 0;
 }
 
 /*
@@ -1014,6 +1028,7 @@ static void handle_stripe(struct stripe_
 			rbi = dev->toread;
 			dev->toread = NULL;
 			spin_unlock_irq(&conf->device_lock);
+			wake_up(&sh->wait);
 			while (rbi && rbi->bi_sector < dev->sector + STRIPE_SECTORS) {
 				copy_data(0, rbi, dev->page, dev->sector);
 				rbi2 = r5_next_bio(rbi, dev->sector);
@@ -1059,6 +1074,7 @@ static void handle_stripe(struct stripe_
 			bi = sh->dev[i].towrite;
 			sh->dev[i].towrite = NULL;
 			if (bi) to_write--;
+			wake_up(&sh->wait);
 
 			while (bi && bi->bi_sector < sh->dev[i].sector + STRIPE_SECTORS){
 				struct bio *nextbi = r5_next_bio(bi, sh->dev[i].sector);
@@ -1511,6 +1527,16 @@ static inline void raid5_plug_device(rai
 	spin_unlock_irq(&conf->device_lock);
 }
 
+static inline void raid5_wait_stripe(struct stripe_head *sh, int dd_idx, int forwrite)
+{
+	struct bio **bip;
+	if (forwrite)
+		bip = &sh->dev[dd_idx].towrite;
+	else
+		bip = &sh->dev[dd_idx].toread;
+	wait_event(sh->wait, *bip == NULL);
+}
+
 static int make_request (request_queue_t *q, struct bio * bi)
 {
 	mddev_t *mddev = q->queuedata;
@@ -1580,6 +1606,7 @@ repeat:
 				 * if we can't, then it's time to submit
 				 * all collected bio's in order to free
 				 * some space in the cache -bzzz */
+try_stripe:
 				sh = get_active_stripe(conf, new_sector, pd_idx, 1);
 				if (!sh && !(bi->bi_rw&RWA_MASK)) {
 					raid5_flush_bios(conf, bios, raid_disks);
@@ -1587,7 +1614,11 @@ repeat:
 				}
 			}
 			if (sh) {
-				add_stripe_bio(sh, bi, dd_idx, (bi->bi_rw&RW_MASK));
+				if (add_stripe_bio(sh, bi, dd_idx, (bi->bi_rw&RW_MASK))) {
+					release_stripe(sh);
+					raid5_wait_stripe(sh, dd_idx, bi->bi_rw&RW_MASK);
+					goto try_stripe;
+				}
 			} else {
 				/* cannot get stripe for read-ahead, just give-up */
 				clear_bit(BIO_UPTODATE, &bi->bi_flags);