drbd: Protect accesses to the uuid set with a spinlock

There is at least the worker context, the receiver context, the context of
receiving netlink packts and processes reading a sysfs attribute that access
the uuids.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_nl.c

diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
index edb490a..ab66055 100644
--- a/drivers/block/drbd/drbd_nl.c
+++ b/drivers/block/drbd/drbd_nl.c
@@ -977,6 +977,8 @@
 	nbc->dc.fencing       = DRBD_FENCING_DEF;
 	nbc->dc.max_bio_bvecs = DRBD_MAX_BIO_BVECS_DEF;
 
+	spin_lock_init(&nbc->md.uuid_lock);
+
 	if (!disk_conf_from_tags(mdev, nlp->tag_list, &nbc->dc)) {
 		retcode = ERR_MANDATORY_TAG;
 		goto fail;
@@ -2170,8 +2172,11 @@
 	tl = reply->tag_list;
 
 	if (get_ldev(mdev)) {
+		unsigned long flags;
+		spin_lock_irqsave(&mdev->ldev->md.uuid_lock, flags);
 		tl = tl_add_blob(tl, T_uuids, mdev->ldev->md.uuid, UI_SIZE*sizeof(u64));
 		tl = tl_add_int(tl, T_uuids_flags, &mdev->ldev->md.flags);
+		spin_unlock_irqrestore(&mdev->ldev->md.uuid_lock, flags);
 		put_ldev(mdev);
 	}
 	put_unaligned(TT_END, tl++); /* Close the tag list */