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rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes()
[ Upstream commit28319d6dc5
] 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:3f95aa81d2
("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>
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3 changed files with 32 additions and 2 deletions
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@ -238,6 +238,7 @@ void synchronize_rcu_tasks_rude(void);
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#define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t, false)
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#define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t, false)
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void exit_tasks_rcu_start(void);
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void exit_tasks_rcu_start(void);
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void exit_tasks_rcu_stop(void);
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void exit_tasks_rcu_finish(void);
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void exit_tasks_rcu_finish(void);
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#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
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#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
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#define rcu_tasks_classic_qs(t, preempt) do { } while (0)
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#define rcu_tasks_classic_qs(t, preempt) do { } while (0)
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@ -246,6 +247,7 @@ void exit_tasks_rcu_finish(void);
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#define call_rcu_tasks call_rcu
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#define call_rcu_tasks call_rcu
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#define synchronize_rcu_tasks synchronize_rcu
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#define synchronize_rcu_tasks synchronize_rcu
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static inline void exit_tasks_rcu_start(void) { }
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static inline void exit_tasks_rcu_start(void) { }
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static inline void exit_tasks_rcu_stop(void) { }
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static inline void exit_tasks_rcu_finish(void) { }
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static inline void exit_tasks_rcu_finish(void) { }
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#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
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#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
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@ -244,7 +244,24 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
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set_current_state(TASK_INTERRUPTIBLE);
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set_current_state(TASK_INTERRUPTIBLE);
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if (pid_ns->pid_allocated == init_pids)
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if (pid_ns->pid_allocated == init_pids)
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break;
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break;
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/*
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* Release tasks_rcu_exit_srcu to avoid following deadlock:
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*
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* 1) TASK A unshare(CLONE_NEWPID)
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* 2) TASK A fork() twice -> TASK B (child reaper for new ns)
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* and TASK C
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* 3) TASK B exits, kills TASK C, waits for TASK A to reap it
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* 4) TASK A calls synchronize_rcu_tasks()
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* -> synchronize_srcu(tasks_rcu_exit_srcu)
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* 5) *DEADLOCK*
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*
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* It is considered safe to release tasks_rcu_exit_srcu here
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* because we assume the current task can not be concurrently
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* reaped at this point.
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*/
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exit_tasks_rcu_stop();
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schedule();
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schedule();
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exit_tasks_rcu_start();
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}
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}
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__set_current_state(TASK_RUNNING);
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__set_current_state(TASK_RUNNING);
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@ -1016,16 +1016,27 @@ void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
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* task is exiting and may be removed from the tasklist. See
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* task is exiting and may be removed from the tasklist. See
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* corresponding synchronize_srcu() for further details.
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* corresponding synchronize_srcu() for further details.
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*/
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*/
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void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)
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void exit_tasks_rcu_stop(void) __releases(&tasks_rcu_exit_srcu)
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{
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{
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struct task_struct *t = current;
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struct task_struct *t = current;
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__srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
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__srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
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exit_tasks_rcu_finish_trace(t);
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}
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/*
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* Contribute to protect against tasklist scan blind spot while the
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* task is exiting and may be removed from the tasklist. See
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* corresponding synchronize_srcu() for further details.
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*/
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void exit_tasks_rcu_finish(void)
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{
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exit_tasks_rcu_stop();
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exit_tasks_rcu_finish_trace(current);
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}
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}
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#else /* #ifdef CONFIG_TASKS_RCU */
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#else /* #ifdef CONFIG_TASKS_RCU */
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void exit_tasks_rcu_start(void) { }
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void exit_tasks_rcu_start(void) { }
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void exit_tasks_rcu_stop(void) { }
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void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
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void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
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#endif /* #else #ifdef CONFIG_TASKS_RCU */
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#endif /* #else #ifdef CONFIG_TASKS_RCU */
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