From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tejun Heo Subject: [PATCH 8/8] cgroup: Add memory barriers to plug cgroup_rstat_updated() race window Date: Fri, 23 Mar 2018 16:13:13 -0700 Message-ID: <20180323231313.1254142-9-tj@kernel.org> References: <20180323231313.1254142-1-tj@kernel.org> Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:from:to:cc:subject:date:message-id:in-reply-to:references; bh=451ZUnYHZwxwIvzxod5IijT9VQcgy3AzhdkdTZSaapw=; b=dWdaIH7DWisCSFxVD3J3HM2fgyEHbO1IYrrnhgKqiE14YLWGUif/f3FFmCG43RNpsX P54vZOe6xQw/gjAWhZaU3iTgdqcJ64oXUmgDcY2N6WCgZtX4eul7pQs2W/dkeTFWHOyW fdaO+KshznUT7hTikk2xQYHFCzLxpfCbqtyU+jN2R+C/X35vtkYY1T0U19CgQNEVbO9U IXBpSkJhUrUg2MIgZ/7BXon57rPOkamQ25NQkCZXLeyuu4hBxP5mv7DtN436vL6WkRKS kVbhIfw8V9ReKiJ6q3HMeCIuaaJDFiuA0butatDsV1Dgdgq4MKIo0L9ErPCU6nlwQhUM 44BQ== In-Reply-To: <20180323231313.1254142-1-tj@kernel.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: lizefan@huawei.com, hannes@cmpxchg.org Cc: cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, guro@fb.com, kernel-team@fb.com, Tejun Heo cgroup_rstat_updated() has a small race window where an updated signaling can race with flush and could be lost till the next update. This wasn't a problem for the existing usages, but we plan to use rstat to track counters which need to be accurate. This patch plugs the race window by synchronizing cgroup_rstat_updated() and flush path with memory barriers around cgroup_rstat_cpu->updated_next pointer. Signed-off-by: Tejun Heo --- kernel/cgroup/rstat.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c index 18b464a..662d7ae 100644 --- a/kernel/cgroup/rstat.c +++ b/kernel/cgroup/rstat.c @@ -28,9 +28,12 @@ void cgroup_rstat_updated(struct cgroup *cgrp, int cpu) unsigned long flags; /* - * Speculative already-on-list test. This may race leading to - * temporary inaccuracies, which is fine. - * + * Paired with the one in cgroup_rstat_cpu_pop_upated(). Either we + * see NULL updated_next or they see our updated stat. + */ + smp_mb(); + + /* * Because @parent's updated_children is terminated with @parent * instead of NULL, we can tell whether @cgrp is on the list by * testing the next pointer for NULL. @@ -125,6 +128,13 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos, *nextp = rstatc->updated_next; rstatc->updated_next = NULL; + + /* + * Paired with the one in cgroup_rstat_cpu_updated(). + * Either they see NULL updated_next or we see their + * updated stat. + */ + smp_mb(); } return pos; -- 2.9.5