Patch "xen/blkback: free requests on disconnection" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

xen/blkback: free requests on disconnection

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
xen-blkback-free-requests-on-disconnection.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Roger Pau Monne <[email protected]>
Date: Fri, 4 Sep 2015 12:08:07 +0200
Subject: xen/blkback: free requests on disconnection
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From: Roger Pau Monne <[email protected]>

commit f929d42ceb18a8acfd47e0e7b7d90b5d49bd9258 upstream.

This is due to commit 86839c56dee28c315a4c19b7bfee450ccd84cd25 "xen/block: add multi-page ring support"

When using an guest under UEFI - after the domain is destroyed the following warning comes from blkback.

------------[ cut here ]------------
WARNING: CPU: 2 PID: 95 at
/home/julien/works/linux/drivers/block/xen-blkback/xenbus.c:274
xen_blkif_deferred_free+0x1f4/0x1f8()
Modules linked in:
CPU: 2 PID: 95 Comm: kworker/2:1 Tainted: G        W       4.2.0 #85
Hardware name: APM X-Gene Mustang board (DT)
Workqueue: events xen_blkif_deferred_free
Call trace:
[<ffff8000000890a8>] dump_backtrace+0x0/0x124
[<ffff8000000891dc>] show_stack+0x10/0x1c
[<ffff8000007653bc>] dump_stack+0x78/0x98
[<ffff800000097e88>] warn_slowpath_common+0x9c/0xd4
[<ffff800000097f80>] warn_slowpath_null+0x14/0x20
[<ffff800000557a0c>] xen_blkif_deferred_free+0x1f0/0x1f8
[<ffff8000000ad020>] process_one_work+0x160/0x3b4
[<ffff8000000ad3b4>] worker_thread+0x140/0x494
[<ffff8000000b2e34>] kthread+0xd8/0xf0
---[ end trace 6f859b7883c88cdd ]---

Request allocation has been moved to connect_ring, which is called every time blkback connects to the frontend (this can happen multiple times during a blkback instance life cycle). On the other hand, request freeing has not been moved, so it's only called when destroying the backend instance. Due to this mismatch, blkback can allocate the request pool multiple times, without freeing it.

In order to fix it, move the freeing of requests to xen_blkif_disconnect to restore the symmetry between request allocation and freeing.

---
 drivers/block/xen-blkback/xenbus.c |   38 +++++++++++++++++++------------------
 1 file changed, 20 insertions(+), 18 deletions(-)

@@ -212,6 +212,9 @@ static int xen_blkif_map(struct xen_blki
 
 static int xen_blkif_disconnect(struct xen_blkif *blkif)
 {
+	struct pending_req *req, *n;
+	int i = 0, j;
+
 	if (blkif->xenblkd) {
 		kthread_stop(blkif->xenblkd);
 		wake_up(&blkif->shutdown_wq);

@@ -238,13 +241,28 @@ static int xen_blkif_disconnect(struct x
 	/* Remove all persistent grants and the cache of ballooned pages. */
 	xen_blkbk_free_caches(blkif);
 
+	/* Check that there is no request in use */
+	list_for_each_entry_safe(req, n, &blkif->pending_free, free_list) {
+		list_del(&req->free_list);
+
+		for (j = 0; j < MAX_INDIRECT_SEGMENTS; j++)
+			kfree(req->segments[j]);
+
+		for (j = 0; j < MAX_INDIRECT_PAGES; j++)
+			kfree(req->indirect_pages[j]);
+
+		kfree(req);
+		i++;
+	}
+
+	WARN_ON(i != (XEN_BLKIF_REQS_PER_PAGE * blkif->nr_ring_pages));
+	blkif->nr_ring_pages = 0;
+
 	return 0;
 }
 
 static void xen_blkif_free(struct xen_blkif *blkif)
 {
-	struct pending_req *req, *n;
-	int i = 0, j;
 
 	xen_blkif_disconnect(blkif);
 	xen_vbd_free(&blkif->vbd);

@@ -257,22 +275,6 @@ static void xen_blkif_free(struct xen_bl
 	BUG_ON(!list_empty(&blkif->free_pages));
 	BUG_ON(!RB_EMPTY_ROOT(&blkif->persistent_gnts));
 
-	/* Check that there is no request in use */
-	list_for_each_entry_safe(req, n, &blkif->pending_free, free_list) {
-		list_del(&req->free_list);
-
-		for (j = 0; j < MAX_INDIRECT_SEGMENTS; j++)
-			kfree(req->segments[j]);
-
-		for (j = 0; j < MAX_INDIRECT_PAGES; j++)
-			kfree(req->indirect_pages[j]);
-
-		kfree(req);
-		i++;
-	}
-
-	WARN_ON(i != (XEN_BLKIF_REQS_PER_PAGE * blkif->nr_ring_pages));
-
 	kmem_cache_free(xen_blkif_cachep, blkif);
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/xen-blkback-free-requests-on-disconnection.patch