From: Frederic Weisbecker frederic@kernel.org
[ 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@intel.com Suggested-by: Boqun Feng boqun.feng@gmail.com Suggested-by: Neeraj Upadhyay quic_neeraju@quicinc.com Suggested-by: Paul E. McKenney paulmck@kernel.org Cc: Oleg Nesterov oleg@redhat.com Cc: Lai Jiangshan jiangshanlai@gmail.com Cc: Eric W . Biederman ebiederm@xmission.com Signed-off-by: Frederic Weisbecker frederic@kernel.org Signed-off-by: Paul E. McKenney paulmck@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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 */