md: support barrier requests on all personalities.

Previously barriers were only supported on RAID1.  This is because
other levels requires synchronisation across all devices and so needed
a different approach.
Here is that approach.

When a barrier arrives, we send a zero-length barrier to every active
device.  When that completes - and if the original request was not
empty -  we submit the barrier request itself (with the barrier flag
cleared) and then submit a fresh load of zero length barriers.

The barrier request itself is asynchronous, but any subsequent
request will block until the barrier completes.

The reason for clearing the barrier flag is that a barrier request is
allowed to fail.  If we pass a non-empty barrier through a striping
raid level it is conceivable that part of it could succeed and part
could fail.  That would be way too hard to deal with.
So if the first run of zero length barriers succeed, we assume all is
sufficiently well that we send the request and ignore errors in the
second run of barriers.

RAID5 needs extra care as write requests may not have been submitted
to the underlying devices yet.  So we flush the stripe cache before
proceeding with the barrier.

Note that the second set of zero-length barriers are submitted
immediately after the original request is submitted.  Thus when
a personality finds mddev->barrier to be set during make_request,
it should not return from make_request until the corresponding
per-device request(s) have been queued.

That will be done in later patches.

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Andre Noll <maan@systemlinux.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/md/md.h b/drivers/md/md.h
index 87430fea287..cb036868a9e 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -292,6 +292,17 @@ struct mddev_s
 	struct mutex			bitmap_mutex;
 
 	struct list_head		all_mddevs;
+
+	/* Generic barrier handling.
+	 * If there is a pending barrier request, all other
+	 * writes are blocked while the devices are flushed.
+	 * The last to finish a flush schedules a worker to
+	 * submit the barrier request (without the barrier flag),
+	 * then submit more flush requests.
+	 */
+	struct bio *barrier;
+	atomic_t flush_pending;
+	struct work_struct barrier_work;
 };
 
 
@@ -432,6 +443,7 @@ extern void md_done_sync(mddev_t *mddev, int blocks, int ok);
 extern void md_error(mddev_t *mddev, mdk_rdev_t *rdev);
 
 extern int mddev_congested(mddev_t *mddev, int bits);
+extern void md_barrier_request(mddev_t *mddev, struct bio *bio);
 extern void md_super_write(mddev_t *mddev, mdk_rdev_t *rdev,
 			   sector_t sector, int size, struct page *page);
 extern void md_super_wait(mddev_t *mddev);