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Revert "rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes()"

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This reverts commit 28319d6dc5. The race
it fixed was subject to conditions that don't exist anymore since:

	1612160b91 ("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: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2024-04-25 16:24:04 +02:00 committed by Paul E. McKenney
parent 1613e604df
commit 9855c37edf
3 changed files with 3 additions and 32 deletions
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2
include/linux/rcupdate.h
View File

@ -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 */

17
kernel/pid_namespace.c
View File

@ -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);

16
kernel/rcu/tasks.h
View File

@ -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 */