6.10-stable review patch. If anyone has any objections, please let me know.
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From: Frederic Weisbecker frederic@kernel.org
[ Upstream commit 9855c37edf0009cc276cecfee09f7e76e2380212 ]
This reverts commit 28319d6dc5e2ffefa452c2377dd0f71621b5bff0. The race it fixed was subject to conditions that don't exist anymore since:
1612160b9127 ("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 Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/rcupdate.h | 2 -- kernel/pid_namespace.c | 17 ----------------- kernel/rcu/tasks.h | 16 +++------------- 3 files changed, 3 insertions(+), 32 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index dfd2399f2cde0..61cb3de236af1 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -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 */
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c index 25f3cf679b358..bdf0087d64423 100644 --- a/kernel/pid_namespace.c +++ b/kernel/pid_namespace.c @@ -249,24 +249,7 @@ 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);
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 098e82bcc427f..ba3440a45b6dd 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -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 */