[3.19.y-ckt stable] Patch "bcache: fix race of writeback thread starting before complete initialization" has been added to the 3.19.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    bcache: fix race of writeback thread starting before complete initialization

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt18.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From b97966cbacce3748f228d366d5affdd2339504c6 Mon Sep 17 00:00:00 2001
From: Eric Wheeler <git at linux.ewheeler.net>
Date: Fri, 26 Feb 2016 14:39:06 -0800
Subject: bcache: fix race of writeback thread starting before complete
 initialization

commit 07cc6ef8edc47f8b4fc1e276d31127a0a5863d4d upstream.

The bch_writeback_thread might BUG_ON in read_dirty() if
dc->sb==BDEV_STATE_DIRTY and bch_sectors_dirty_init has not yet completed
its related initialization.  This patch downs the dc->writeback_lock until
after initialization is complete, thus preventing bch_writeback_thread
from proceeding prematurely.

See this thread:
  http://thread.gmane.org/gmane.linux.kernel.bcache.devel/3453

Signed-off-by: Eric Wheeler <bcache at linux.ewheeler.net>
Tested-by: Marc MERLIN <marc at merlins.org>
Signed-off-by: Jens Axboe <axboe at fb.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/md/bcache/super.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 42522c8..e83629a 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1046,8 +1046,12 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c)
 	 */
 	atomic_set(&dc->count, 1);

-	if (bch_cached_dev_writeback_start(dc))
+	/* Block writeback thread, but spawn it */
+	down_write(&dc->writeback_lock);
+	if (bch_cached_dev_writeback_start(dc)) {
+		up_write(&dc->writeback_lock);
 		return -ENOMEM;
+	}

 	if (BDEV_STATE(&dc->sb) == BDEV_STATE_DIRTY) {
 		bch_sectors_dirty_init(dc);
@@ -1059,6 +1063,9 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c)
 	bch_cached_dev_run(dc);
 	bcache_device_link(&dc->disk, c, "bdev");

+	/* Allow the writeback thread to proceed */
+	up_write(&dc->writeback_lock);
+
 	pr_info("Caching %s as %s on set %pU",
 		bdevname(dc->bdev, buf), dc->disk.disk->disk_name,
 		dc->disk.c->sb.set_uuid);
--
2.7.4