Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
CC:     Prateek Sood <prsood@codeaurora.org>, Tejun Heo <tj@kernel.org>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.14 030/110] cgroup: Fix deadlock in cpu hotplug
 path
Thread-Topic: [PATCH AUTOSEL for 4.14 030/110] cgroup: Fix deadlock in cpu
 hotplug path
Thread-Index: AQHTnRjoWBzwV/tJq0uAU/xdZ9hS9g==
Date:   Sat, 3 Feb 2018 18:00:42 +0000
Message-ID: <20180203180015.29073-30-alexander.levin@microsoft.com>
References: <20180203180015.29073-1-alexander.levin@microsoft.com>
In-Reply-To: <20180203180015.29073-1-alexander.levin@microsoft.com>
Accept-Language: en-US
Content-Language: en-US
received-spf: None (protection.outlook.com: microsoft.com does not designate
 permitted sender hosts)
spamdiagnosticoutput: 1:99
spamdiagnosticmetadata: NSPM
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-MS-Exchange-CrossTenant-Network-Message-Id: a045669a-bb4c-4499-c5b6-08d56b300c2d
X-MS-Exchange-CrossTenant-originalarrivaltime: 03 Feb 2018 18:00:42.6753
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: 72f988bf-86f1-41af-91ab-2d7cd011db47
X-MS-Exchange-Transport-CrossTenantHeadersStamped: BL0PR2101MB1090
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

From: Prateek Sood <prsood@codeaurora.org>

[ Upstream commit 116d2f7496c51b2e02e8e4ecdd2bdf5fb9d5a641 ]

Deadlock during cgroup migration from cpu hotplug path when a task T is
being moved from source to destination cgroup.

kworker/0:0
cpuset_hotplug_workfn()
   cpuset_hotplug_update_tasks()
      hotplug_update_tasks_legacy()
        remove_tasks_in_empty_cpuset()
          cgroup_transfer_tasks() // stuck in iterator loop
            cgroup_migrate()
              cgroup_migrate_add_task()

In cgroup_migrate_add_task() it checks for PF_EXITING flag of task T.
Task T will not migrate to destination cgroup. css_task_iter_start()
will keep pointing to task T in loop waiting for task T cg_list node
to be removed.

Task T
do_exit()
  exit_signals() // sets PF_EXITING
  exit_task_namespaces()
    switch_task_namespaces()
      free_nsproxy()
        put_mnt_ns()
          drop_collected_mounts()
            namespace_unlock()
              synchronize_rcu()
                _synchronize_rcu_expedited()
                  schedule_work() // on cpu0 low priority worker pool
                  wait_event() // waiting for work item to execute

Task T inserted a work item in the worklist of cpu0 low priority
worker pool. It is waiting for expedited grace period work item
to execute. This work item will only be executed once kworker/0:0
complete execution of cpuset_hotplug_workfn().

kworker/0:0 =3D=3D> Task T =3D=3D>kworker/0:0

In case of PF_EXITING task being migrated from source to destination
cgroup, migrate next available task in source cgroup.

Signed-off-by: Prateek Sood <prsood@codeaurora.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 kernel/cgroup/cgroup-v1.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 024085daab1a..a2c05d2476ac 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -123,7 +123,11 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cg=
roup *from)
 	 */
 	do {
 		css_task_iter_start(&from->self, 0, &it);
-		task =3D css_task_iter_next(&it);
+
+		do {
+			task =3D css_task_iter_next(&it);
+		} while (task && (task->flags & PF_EXITING));
+
 		if (task)
 			get_task_struct(task);
 		css_task_iter_end(&it);
--=20
2.11.0