[PATCH] md: Fix issues with referencing rdev in md/raid1

We need to be careful when referencing mirrors[i].rdev. It can disappear
under us at various times.

So:
fix a couple of problem places.
comment a couple of non-problem places
move an 'atomic_add' which deferences rdev down a little
way to some where where it is sure to not be NULL.

Signed-off-by: Neil Brown <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/md/raid1.c

···
 
 	seq_printf(seq, " [%d/%d] [", conf->raid_disks,
 						conf->working_disks);
-	for (i = 0; i < conf->raid_disks; i++)
+	rcu_read_lock();
+	for (i = 0; i < conf->raid_disks; i++) {
+		mdk_rdev_t *rdev = rcu_dereference(conf->mirrors[i].rdev);
 		seq_printf(seq, "%s",
-			      conf->mirrors[i].rdev &&
-			      test_bit(In_sync, &conf->mirrors[i].rdev->flags) ? "U" : "_");
+			   rdev && test_bit(In_sync, &rdev->flags) ? "U" : "_");
+	}
+	rcu_read_unlock();
 	seq_printf(seq, "]");
 }
 
···
 static void print_conf(conf_t *conf)
 {
 	int i;
-	mirror_info_t *tmp;
 
 	printk("RAID1 conf printout:\n");
 	if (!conf) {
···
 	printk(" --- wd:%d rd:%d\n", conf->working_disks,
 		conf->raid_disks);
 
+	rcu_read_lock();
 	for (i = 0; i < conf->raid_disks; i++) {
 		char b[BDEVNAME_SIZE];
-		tmp = conf->mirrors + i;
-		if (tmp->rdev)
+		mdk_rdev_t *rdev = rcu_dereference(conf->mirrors[i].rdev);
+		if (rdev)
 			printk(" disk %d, wo:%d, o:%d, dev:%s\n",
-				i, !test_bit(In_sync, &tmp->rdev->flags), !test_bit(Faulty, &tmp->rdev->flags),
-				bdevname(tmp->rdev->bdev,b));
+			       i, !test_bit(In_sync, &rdev->flags),
+			       !test_bit(Faulty, &rdev->flags),
+			       bdevname(rdev->bdev,b));
 	}
+	rcu_read_unlock();
 }
 
 static void close_sync(conf_t *conf)
···
 {
 	int i;
 	conf_t *conf = mddev->private;
-	mirror_info_t *tmp;
 
 	/*
 	 * Find all failed disks within the RAID1 configuration 
-	 * and mark them readable
+	 * and mark them readable.
+	 * Called under mddev lock, so rcu protection not needed.
 	 */
 	for (i = 0; i < conf->raid_disks; i++) {
-		tmp = conf->mirrors + i;
-		if (tmp->rdev 
-		    && !test_bit(Faulty, &tmp->rdev->flags)
-		    && !test_bit(In_sync, &tmp->rdev->flags)) {
+		mdk_rdev_t *rdev = conf->mirrors[i].rdev;
+		if (rdev
+		    && !test_bit(Faulty, &rdev->flags)
+		    && !test_bit(In_sync, &rdev->flags)) {
 			conf->working_disks++;
 			mddev->degraded--;
-			set_bit(In_sync, &tmp->rdev->flags);
+			set_bit(In_sync, &rdev->flags);
 		}
 	}
 
···
 		/* ouch - failed to read all of that.
 		 * Try some synchronous reads of other devices to get
 		 * good data, much like with normal read errors.  Only
-		 * read into the pages we already have so they we don't
+		 * read into the pages we already have so we don't
 		 * need to re-issue the read request.
 		 * We don't need to freeze the array, because being in an
 		 * active sync request, there is no normal IO, and
···
 				s = PAGE_SIZE >> 9;
 			do {
 				if (r1_bio->bios[d]->bi_end_io == end_sync_read) {
+					/* No rcu protection needed here devices
+					 * can only be removed when no resync is
+					 * active, and resync is currently active
+					 */
 					rdev = conf->mirrors[d].rdev;
 					if (sync_page_io(rdev->bdev,
 							 sect + rdev->data_offset,
···
 					s = PAGE_SIZE >> 9;
 
 				do {
+					/* Note: no rcu protection needed here
+					 * as this is synchronous in the raid1d thread
+					 * which is the thread that might remove
+					 * a device.  If raid1d ever becomes multi-threaded....
+					 */
 					rdev = conf->mirrors[d].rdev;
 					if (rdev &&
 					    test_bit(In_sync, &rdev->flags) &&
···
 							d = conf->raid_disks;
 						d--;
 						rdev = conf->mirrors[d].rdev;
-						atomic_add(s, &rdev->corrected_errors);
 						if (rdev &&
 						    test_bit(In_sync, &rdev->flags)) {
 							if (sync_page_io(rdev->bdev,
···
 									 s<<9, conf->tmppage, READ) == 0)
 								/* Well, this device is dead */
 								md_error(mddev, rdev);
-							else
+							else {
+								atomic_add(s, &rdev->corrected_errors);
 								printk(KERN_INFO "raid1:%s: read error corrected (%d sectors at %llu on %s)\n",
 								       mdname(mddev), s, (unsigned long long)(sect + rdev->data_offset), bdevname(rdev->bdev, b));
+							}
 						}
 					}
 				} else {
···
 		for (i=0; i<conf->raid_disks; i++) {
 			bio = r1_bio->bios[i];
 			if (bio->bi_end_io == end_sync_read) {
-				md_sync_acct(conf->mirrors[i].rdev->bdev, nr_sectors);
+				md_sync_acct(bio->bi_bdev, nr_sectors);
 				generic_make_request(bio);
 			}
 		}
 	} else {
 		atomic_set(&r1_bio->remaining, 1);
 		bio = r1_bio->bios[r1_bio->read_disk];
-		md_sync_acct(conf->mirrors[r1_bio->read_disk].rdev->bdev,
-			     nr_sectors);
+		md_sync_acct(bio->bi_bdev, nr_sectors);
 		generic_make_request(bio);
 
 	}
-
 	return nr_sectors;
 }