* [PATCH] cgroup: Fix potential race between cgroup_exit and cpuset_attach
@ 2017-08-30 12:53 Neeraj Upadhyay
2017-08-31 1:10 ` Tejun Heo
0 siblings, 1 reply; 2+ messages in thread
From: Neeraj Upadhyay @ 2017-08-30 12:53 UTC (permalink / raw)
To: tj, lizefan, mingo, longman, akpm
Cc: linux-kernel, sramana, prsood, Neeraj Upadhyay
There is a potential race between cgroup_exit() and the taskset
migration path. This race happens when all tasks associated
with the cg_list entries in mg_tasks, detach themselves
before the tasks can be attached to the destination cpuset in
cpuset_attach(). Below is the sequence where race is observed:
cpuset_hotplug_workfn()
cgroup_transfer_tasks()
cgroup_migrate()
cgroup_migrate_execute()
<TASK EXIT>
list_del_init(&task->cg_list)
cpuset_attach()
cgroup_taskset_first(tset, &css) // css is not set
guarantee_online_mems(cs, ...) // data abort
Fix this by adding a checking to verify that css is set from
cgroup_taskset_first(), before proceeding.
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
---
Hi,
We observed this issue for cgroup code corresponding to stable
v4.4.85 snapshot 3144d81 ("cgroup, kthread: close race window where
new kthreads can be migrated to non-root cgroups"). Can you please
tell us, if there are any patches in latest code, which
fixes these issue?
kernel/cgroup/cpuset.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 87a1213..7e245f26 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1510,11 +1510,17 @@ static void cpuset_attach(struct cgroup_taskset *tset)
static nodemask_t cpuset_attach_nodemask_to;
struct task_struct *task;
struct task_struct *leader;
- struct cgroup_subsys_state *css;
+ struct cgroup_subsys_state *css = NULL;
struct cpuset *cs;
struct cpuset *oldcs = cpuset_attach_old_cs;
cgroup_taskset_first(tset, &css);
+ /* If all mg_tasks detached (from cgroup_exit())
+ * before we started scanning the mg_tasks in
+ * cgroup_taskset_next().
+ */
+ if (css == NULL)
+ return;
cs = css_cs(css);
mutex_lock(&cpuset_mutex);
--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] cgroup: Fix potential race between cgroup_exit and cpuset_attach
2017-08-30 12:53 [PATCH] cgroup: Fix potential race between cgroup_exit and cpuset_attach Neeraj Upadhyay
@ 2017-08-31 1:10 ` Tejun Heo
0 siblings, 0 replies; 2+ messages in thread
From: Tejun Heo @ 2017-08-31 1:10 UTC (permalink / raw)
To: Neeraj Upadhyay
Cc: lizefan, mingo, longman, akpm, linux-kernel, sramana, prsood
Hello,
On Wed, Aug 30, 2017 at 06:23:50PM +0530, Neeraj Upadhyay wrote:
> cpuset_hotplug_workfn()
> cgroup_transfer_tasks()
> cgroup_migrate()
> cgroup_migrate_execute()
> <TASK EXIT>
> list_del_init(&task->cg_list)
> cpuset_attach()
> cgroup_taskset_first(tset, &css) // css is not set
> guarantee_online_mems(cs, ...) // data abort
I don't think this can happen anymore. We lock out task exits during
migration and don't call migration callbacks if there's no tasks to
migrate.
Thanks.
--
tejun
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2017-08-31 1:10 ` Tejun Heo
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