From mboxrd@z Thu Jan 1 00:00:00 1970 From: Martijn Coenen Subject: [PATCH] memcg: Only free spare array when readers are done Date: Thu, 14 Jan 2016 14:33:52 +0100 Message-ID: <001a113abaa499606605294b5b17@google.com> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20120113; h=mime-version:date:message-id:subject:from:to:cc:content-type; bh=F045+Kud0Qdy41oKGEv5jGz4PK0qEbBOdZdLblbc5KY=; b=BTIgWi5/vdRrRWysDx9GaRH75wRcVHyXmLjS3gTJf6Xy5kQ1WRchjYGB1d2M49M2do tsNCgqW+E7K89XTqDjoZzJAoGrzGj1TeJF674aM626sCdOi4BOSIVn1SPBxTph5REJwa DHudJL+BpuF6XYQGW3a9F0XxVAtgVtdv1i0Ta181w0q8gk8Y6T2/gSuXlh6TiTUS+c18 AqelxqnpL0kle0XXFNEyQ5//EF3YBRRmR2UGAovyToXtvlfooLNa4U5KGd+kdqdqugFu zLIoIjMch5dkoIkIAVSeNyZIZRCRdOaMTAROoSeBRYwSFBg6kfsfWFDAOON3R6C/ntVI Eolg== Sender: owner-linux-mm@kvack.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed"; delsp="yes" Content-Transfer-Encoding: 7bit To: Johannes Weiner Cc: Michal Hocko , Vladimir Davydov , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org 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(). Signed-off-by: Martijn Coenen --- mm/memcontrol.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 14cb1db..73228b6 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3522,16 +3522,17 @@ static void __mem_cgroup_usage_unregister_event(struct mem_cgroup *memcg, 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); } -- 2.6.0.rc2.230.g3dd15c0 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753687AbcANNnM (ORCPT ); Thu, 14 Jan 2016 08:43:12 -0500 Received: from mail-qk0-f201.google.com ([209.85.220.201]:33331 "EHLO mail-qk0-f201.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750969AbcANNnK (ORCPT ); Thu, 14 Jan 2016 08:43:10 -0500 MIME-Version: 1.0 Date: Thu, 14 Jan 2016 14:33:52 +0100 Message-ID: <001a113abaa499606605294b5b17@google.com> Subject: [PATCH] memcg: Only free spare array when readers are done From: Martijn Coenen To: Johannes Weiner Cc: Michal Hocko , Vladimir Davydov , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset=UTF-8; format=flowed; delsp=yes Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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(). Signed-off-by: Martijn Coenen --- mm/memcontrol.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 14cb1db..73228b6 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3522,16 +3522,17 @@ static void __mem_cgroup_usage_unregister_event(struct mem_cgroup *memcg, 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); } -- 2.6.0.rc2.230.g3dd15c0