From: Frederic Weisbecker <frederic@xxxxxxxxxx>
[ Upstream commit 28319d6dc5e2ffefa452c2377dd0f71621b5bff0 ]
RCU Tasks and PID-namespace unshare can interact in do_exit() in a
complicated circular dependency:
1) TASK A calls unshare(CLONE_NEWPID), this creates a new PID namespace
that every subsequent child of TASK A will belong to. But TASK A
doesn't itself belong to that new PID namespace.
2) TASK A forks() and creates TASK B. TASK A stays attached to its PID
namespace (let's say PID_NS1) and TASK B is the first task belonging
to the new PID namespace created by unshare() (let's call it PID_NS2).
3) Since TASK B is the first task attached to PID_NS2, it becomes the
PID_NS2 child reaper.
4) TASK A forks() again and creates TASK C which get attached to PID_NS2.
Note how TASK C has TASK A as a parent (belonging to PID_NS1) but has
TASK B (belonging to PID_NS2) as a pid_namespace child_reaper.
5) TASK B exits and since it is the child reaper for PID_NS2, it has to
kill all other tasks attached to PID_NS2, and wait for all of them to
die before getting reaped itself (zap_pid_ns_process()).
6) TASK A calls synchronize_rcu_tasks() which leads to
synchronize_srcu(&tasks_rcu_exit_srcu).
7) TASK B is waiting for TASK C to get reaped. But TASK B is under a
tasks_rcu_exit_srcu SRCU critical section (exit_notify() is between
exit_tasks_rcu_start() and exit_tasks_rcu_finish()), blocking TASK A.
8) TASK C exits and since TASK A is its parent, it waits for it to reap
TASK C, but it can't because TASK A waits for TASK B that waits for
TASK C.
Pid_namespace semantics can hardly be changed at this point. But the
coverage of tasks_rcu_exit_srcu can be reduced instead.
The current task is assumed not to be concurrently reapable at this
stage of exit_notify() and therefore tasks_rcu_exit_srcu can be
temporarily relaxed without breaking its constraints, providing a way
out of the deadlock scenario.
[ paulmck: Fix build failure by adding additional declaration. ]
Fixes: 3f95aa81d265 ("rcu: Make TASKS_RCU handle tasks that are almost done exiting")
Reported-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Suggested-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Suggested-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Eric W . Biederman <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
include/linux/rcupdate.h | 2 ++
kernel/pid_namespace.c | 17 +++++++++++++++++
kernel/rcu/tasks.h | 15 +++++++++++++--
3 files changed, 32 insertions(+), 2 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 095b3b39bd032..1f46db38d6ec6 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -189,6 +189,7 @@ void synchronize_rcu_tasks_rude(void);
#define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t, false)
void exit_tasks_rcu_start(void);
+void exit_tasks_rcu_stop(void);
void exit_tasks_rcu_finish(void);
#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
#define rcu_tasks_qs(t, preempt) do { } while (0)
@@ -196,6 +197,7 @@ void exit_tasks_rcu_finish(void);
#define call_rcu_tasks call_rcu
#define synchronize_rcu_tasks synchronize_rcu
static inline void exit_tasks_rcu_start(void) { }
+static inline void exit_tasks_rcu_stop(void) { }
static inline void exit_tasks_rcu_finish(void) { }
#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index ef8733e2a476e..20243682e6056 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -251,7 +251,24 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
set_current_state(TASK_INTERRUPTIBLE);
if (pid_ns->pid_allocated == init_pids)
break;
+ /*
+ * Release tasks_rcu_exit_srcu to avoid following deadlock:
+ *
+ * 1) TASK A unshare(CLONE_NEWPID)
+ * 2) TASK A fork() twice -> TASK B (child reaper for new ns)
+ * and TASK C
+ * 3) TASK B exits, kills TASK C, waits for TASK A to reap it
+ * 4) TASK A calls synchronize_rcu_tasks()
+ * -> synchronize_srcu(tasks_rcu_exit_srcu)
+ * 5) *DEADLOCK*
+ *
+ * It is considered safe to release tasks_rcu_exit_srcu here
+ * because we assume the current task can not be concurrently
+ * reaped at this point.
+ */
+ exit_tasks_rcu_stop();
schedule();
+ exit_tasks_rcu_start();
}
__set_current_state(TASK_RUNNING);
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index c4af3c05663f4..df8143c8a6a8e 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -604,17 +604,28 @@ void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
* task is exiting and may be removed from the tasklist. See
* corresponding synchronize_srcu() for further details.
*/
-void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)
+void exit_tasks_rcu_stop(void) __releases(&tasks_rcu_exit_srcu)
{
struct task_struct *t = current;
__srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
- exit_tasks_rcu_finish_trace(t);
+}
+
+/*
+ * Contribute to protect against tasklist scan blind spot while the
+ * task is exiting and may be removed from the tasklist. See
+ * corresponding synchronize_srcu() for further details.
+ */
+void exit_tasks_rcu_finish(void)
+{
+ exit_tasks_rcu_stop();
+ exit_tasks_rcu_finish_trace(current);
}
#else /* #ifdef CONFIG_TASKS_RCU */
static inline void show_rcu_tasks_classic_gp_kthread(void) { }
void exit_tasks_rcu_start(void) { }
+void exit_tasks_rcu_stop(void) { }
void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
#endif /* #else #ifdef CONFIG_TASKS_RCU */
--
2.39.2
