[3.16.y-ckt stable] Patch "rbd: fix rbd_dev_parent_get() when parent_overlap == 0" has been added to staging queue

[Luis Henriques]

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

    rbd: fix rbd_dev_parent_get() when parent_overlap == 0

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt6.

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

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 8f86365e0aa989bc0a8b23518b9f846fcd8a22fb Mon Sep 17 00:00:00 2001
From: Ilya Dryomov <idryomov at redhat.com>
Date: Mon, 19 Jan 2015 18:13:43 +0300
Subject: rbd: fix rbd_dev_parent_get() when parent_overlap == 0

commit ae43e9d05eb4bd324155292f889fbd001c4faea8 upstream.

The comment for rbd_dev_parent_get() said

    * We must get the reference before checking for the overlap to
    * coordinate properly with zeroing the parent overlap in
    * rbd_dev_v2_parent_info() when an image gets flattened.  We
    * drop it again if there is no overlap.

but the "drop it again if there is no overlap" part was missing from
the implementation.  This lead to absurd parent_ref values for images
with parent_overlap == 0, as parent_ref was incremented for each
img_request and virtually never decremented.

Fix this by leveraging the fact that refresh path calls
rbd_dev_v2_parent_info() under header_rwsem and use it for read in
rbd_dev_parent_get(), instead of messing around with atomics.  Get rid
of barriers in rbd_dev_v2_parent_info() while at it - I don't see what
they'd pair with now and I suspect we are in a pretty miserable
situation as far as proper locking goes regardless.

Signed-off-by: Ilya Dryomov <idryomov at redhat.com>
Reviewed-by: Josh Durgin <jdurgin at redhat.com>
Reviewed-by: Alex Elder <elder at linaro.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/block/rbd.c | 20 ++++++--------------
 1 file changed, 6 insertions(+), 14 deletions(-)

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index 3d2b99a15f62..28397fc32a85 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -2009,32 +2009,26 @@ static void rbd_dev_parent_put(struct rbd_device *rbd_dev)
  * If an image has a non-zero parent overlap, get a reference to its
  * parent.
  *
- * We must get the reference before checking for the overlap to
- * coordinate properly with zeroing the parent overlap in
- * rbd_dev_v2_parent_info() when an image gets flattened.  We
- * drop it again if there is no overlap.
- *
  * Returns true if the rbd device has a parent with a non-zero
  * overlap and a reference for it was successfully taken, or
  * false otherwise.
  */
 static bool rbd_dev_parent_get(struct rbd_device *rbd_dev)
 {
-	int counter;
+	int counter = 0;

 	if (!rbd_dev->parent_spec)
 		return false;

-	counter = atomic_inc_return_safe(&rbd_dev->parent_ref);
-	if (counter > 0 && rbd_dev->parent_overlap)
-		return true;
-
-	/* Image was flattened, but parent is not yet torn down */
+	down_read(&rbd_dev->header_rwsem);
+	if (rbd_dev->parent_overlap)
+		counter = atomic_inc_return_safe(&rbd_dev->parent_ref);
+	up_read(&rbd_dev->header_rwsem);

 	if (counter < 0)
 		rbd_warn(rbd_dev, "parent reference overflow\n");

-	return false;
+	return counter > 0;
 }

 /*
@@ -4079,7 +4073,6 @@ static int rbd_dev_v2_parent_info(struct rbd_device *rbd_dev)
 		 */
 		if (rbd_dev->parent_overlap) {
 			rbd_dev->parent_overlap = 0;
-			smp_mb();
 			rbd_dev_parent_put(rbd_dev);
 			pr_info("%s: clone image has been flattened\n",
 				rbd_dev->disk->disk_name);
@@ -4123,7 +4116,6 @@ static int rbd_dev_v2_parent_info(struct rbd_device *rbd_dev)
 	 * treat it specially.
 	 */
 	rbd_dev->parent_overlap = overlap;
-	smp_mb();
 	if (!overlap) {

 		/* A null parent_spec indicates it's the initial probe */
--
2.1.4