md linear: Protecting mddev with rcu locks to avoid races

Due to the lack of memory ordering guarantees, we may have races around
mddev->conf.

In particular, the correct contents of the structure we get from
dereferencing ->private might not be visible to this CPU yet, and
they might not be correct w.r.t mddev->raid_disks.

This patch addresses the problem using rcu protection to avoid
such race conditions.

Signed-off-by: SandeepKsinha <sandeepksinha@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/linear.c

@@ -28,10 +28,11 @@
 static inline dev_info_t *which_dev(mddev_t *mddev, sector_t sector)
 {
 	int lo, mid, hi;
-	linear_conf_t *conf = mddev->private;
+	linear_conf_t *conf;
 
 	lo = 0;
 	hi = mddev->raid_disks - 1;
+	conf = rcu_dereference(mddev->private);
 
 	/*
 	 * Binary Search
@@ -66,8 +67,10 @@ static int linear_mergeable_bvec(struct request_queue *q,
 	unsigned long maxsectors, bio_sectors = bvm->bi_size >> 9;
 	sector_t sector = bvm->bi_sector + get_start_sect(bvm->bi_bdev);
 
+	rcu_read_lock();
 	dev0 = which_dev(mddev, sector);
 	maxsectors = dev0->end_sector - sector;
+	rcu_read_unlock();
 
 	if (maxsectors < bio_sectors)
 		maxsectors = 0;
@@ -86,36 +89,50 @@ static int linear_mergeable_bvec(struct request_queue *q,
 static void linear_unplug(struct request_queue *q)
 {
 	mddev_t *mddev = q->queuedata;
-	linear_conf_t *conf = mddev->private;
+	linear_conf_t *conf;
 	int i;
 
+	rcu_read_lock();
+	conf = rcu_dereference(mddev->private);
+
 	for (i=0; i < mddev->raid_disks; i++) {
 		struct request_queue *r_queue = bdev_get_queue(conf->disks[i].rdev->bdev);
 		blk_unplug(r_queue);
 	}
+	rcu_read_unlock();
 }
 
 static int linear_congested(void *data, int bits)
 {
 	mddev_t *mddev = data;
-	linear_conf_t *conf = mddev->private;
+	linear_conf_t *conf;
 	int i, ret = 0;
 
+	rcu_read_lock();
+	conf = rcu_dereference(mddev->private);
+
 	for (i = 0; i < mddev->raid_disks && !ret ; i++) {
 		struct request_queue *q = bdev_get_queue(conf->disks[i].rdev->bdev);
 		ret |= bdi_congested(&q->backing_dev_info, bits);
 	}
+
+	rcu_read_unlock();
 	return ret;
 }
 
 static sector_t linear_size(mddev_t *mddev, sector_t sectors, int raid_disks)
 {
-	linear_conf_t *conf = mddev->private;
+	linear_conf_t *conf;
+	sector_t array_sectors;
 
+	rcu_read_lock();
+	conf = rcu_dereference(mddev->private);
 	WARN_ONCE(sectors || raid_disks,
 		  "%s does not support generic reshape\n", __func__);
+	array_sectors = conf->array_sectors;
+	rcu_read_unlock();
 
-	return conf->array_sectors;
+	return array_sectors;
 }
 
 static linear_conf_t *linear_conf(mddev_t *mddev, int raid_disks)
@@ -229,8 +246,8 @@ static int linear_add(mddev_t *mddev, mdk_rdev_t *rdev)
 		return -ENOMEM;
 
 	newconf->prev = mddev->private;
-	mddev->private = newconf;
 	mddev->raid_disks++;
+	rcu_assign_pointer(mddev->private, newconf);
 	md_set_array_sectors(mddev, linear_size(mddev, 0, 0));
 	set_capacity(mddev->gendisk, mddev->array_sectors);
 	return 0;
@@ -239,7 +256,13 @@ static int linear_add(mddev_t *mddev, mdk_rdev_t *rdev)
 static int linear_stop (mddev_t *mddev)
 {
 	linear_conf_t *conf = mddev->private;
-  
+
+	/*
+	 * We do not require rcu protection here since
+	 * we hold reconfig_mutex for both linear_add and
+	 * linear_stop, so they cannot race.
+	 */
+
 	blk_sync_queue(mddev->queue); /* the unplug fn references 'conf'*/
 	do {
 		linear_conf_t *t = conf->prev;
@@ -269,9 +292,11 @@ static int linear_make_request (struct request_queue *q, struct bio *bio)
 		      bio_sectors(bio));
 	part_stat_unlock();
 
+	rcu_read_lock();
 	tmp_dev = which_dev(mddev, bio->bi_sector);
 	start_sector = tmp_dev->end_sector - tmp_dev->rdev->sectors;
 
+
 	if (unlikely(bio->bi_sector >= (tmp_dev->end_sector)
 		     || (bio->bi_sector < start_sector))) {
 		char b[BDEVNAME_SIZE];
@@ -282,6 +307,7 @@ static int linear_make_request (struct request_queue *q, struct bio *bio)
 			bdevname(tmp_dev->rdev->bdev, b),
 			(unsigned long long)tmp_dev->rdev->sectors,
 			(unsigned long long)start_sector);
+		rcu_read_unlock();
 		bio_io_error(bio);
 		return 0;
 	}
@@ -291,9 +317,11 @@ static int linear_make_request (struct request_queue *q, struct bio *bio)
 		 * split it.
 		 */
 		struct bio_pair *bp;
+		sector_t end_sector = tmp_dev->end_sector;
+
+		rcu_read_unlock();
 
-		bp = bio_split(bio,
-			       tmp_dev->end_sector - bio->bi_sector);
+		bp = bio_split(bio, end_sector - bio->bi_sector);
 
 		if (linear_make_request(q, &bp->bio1))
 			generic_make_request(&bp->bio1);
@@ -306,6 +334,7 @@ static int linear_make_request (struct request_queue *q, struct bio *bio)
 	bio->bi_bdev = tmp_dev->rdev->bdev;
 	bio->bi_sector = bio->bi_sector - start_sector
 		+ tmp_dev->rdev->data_offset;
+	rcu_read_unlock();
 
 	return 1;
 }