Patch "memcg: only free spare array when readers are done" has been added to the 4.4-stable tree

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

memcg: only free spare array when readers are done

to the 4.4-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:
memcg-only-free-spare-array-when-readers-are-done.patch
and it can be found in the queue-4.4 subdirectory.

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

From: Martijn Coenen <[email protected]>
Date: Fri, 15 Jan 2016 16:57:49 -0800
Subject: memcg: only free spare array when readers are done

From: Martijn Coenen <[email protected]>

commit 6611d8d76132f86faa501de9451a89bf23fb2371 upstream.

A spare array holding mem cgroup threshold events is kept around to make sure we can always safely deregister an event and have an array to store the new set of events in.

In the scenario where we're going from 1 to 0 registered events, the pointer to the primary array containing 1 event is copied to the spare slot, and then the spare slot is freed because no events are left. However, it is freed before calling synchronize_rcu(), which means readers may still be accessing threshold->primary after it is freed.

Fixed by only freeing after synchronize_rcu().

---
 mm/memcontrol.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

@@ -3522,16 +3522,17 @@ static void __mem_cgroup_usage_unregiste
 swap_buffers:
 	/* Swap primary and spare array */
 	thresholds->spare = thresholds->primary;
-	/* If all events are unregistered, free the spare array */
-	if (!new) {
-		kfree(thresholds->spare);
-		thresholds->spare = NULL;
-	}
 
 	rcu_assign_pointer(thresholds->primary, new);
 
 	/* To be sure that nobody uses thresholds */
 	synchronize_rcu();
+
+	/* If all events are unregistered, free the spare array */
+	if (!new) {
+		kfree(thresholds->spare);
+		thresholds->spare = NULL;
+	}
 unlock:
 	mutex_unlock(&memcg->thresholds_lock);
 }

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

queue-4.4/memcg-only-free-spare-array-when-readers-are-done.patch