Commit 9855c37e authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Paul E. McKenney
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Revert "rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes()" 



This reverts commit 28319d6d. The race
it fixed was subject to conditions that don't exist anymore since:

	1612160b ("rcu-tasks: Eliminate deadlocks involving do_exit() and RCU tasks")

This latter commit removes the use of SRCU that used to cover the
RCU-tasks blind spot on exit between the tasklist's removal and the
final preemption disabling. The task is now placed instead into a
temporary list inside which voluntary sleeps are accounted as RCU-tasks
quiescent states. This would disarm the deadlock initially reported
against PID namespace exit.

Signed-off-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Reviewed-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 1613e604

include/linux/rcupdate.h

+0 −2
Original line number Diff line number Diff line
@@ -209,7 +209,6 @@ 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_classic_qs(t, preempt) do { } while (0)
@@ -218,7 +217,6 @@ 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 */

kernel/pid_namespace.c

+0 −17
Original line number Diff line number Diff line
@@ -248,24 +248,7 @@ 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);

kernel/rcu/tasks.h

+3 −13
Original line number Diff line number Diff line
@@ -858,7 +858,7 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp) 
//	not know to synchronize with this RCU Tasks grace period) have

//	completed exiting.  The synchronize_rcu() in rcu_tasks_postgp()

//	will take care of any tasks stuck in the non-preemptible region

//	of do_exit() following its call to exit_tasks_rcu_stop().

//	of do_exit() following its call to exit_tasks_rcu_finish().

// check_all_holdout_tasks(), repeatedly until holdout list is empty:

//	Scans the holdout list, attempting to identify a quiescent state

//	for each task on the list.  If there is a quiescent state, the
@@ -1220,7 +1220,7 @@ void exit_tasks_rcu_start(void) 
 * Remove the task from the "yet another list" because do_exit() is now

 * non-preemptible, allowing synchronize_rcu() to wait beyond this point.

 */

void exit_tasks_rcu_stop(void)

void exit_tasks_rcu_finish(void)

{

	unsigned long flags;

	struct rcu_tasks_percpu *rtpcp;
@@ -1231,22 +1231,12 @@ void exit_tasks_rcu_stop(void) 
	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);

	list_del_init(&t->rcu_tasks_exit_list);

	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);

}



/*

 * 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);

	exit_tasks_rcu_finish_trace(t);

}



#else /* #ifdef CONFIG_TASKS_RCU */

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 */