[patch 7/8] posix-cpu-timers: Prevent enqueueing when PF_EXITING is set

[Thomas Gleixner <tglx@kernel.org>]

To prepare for cleaning up POSIX CPU timers in do_exit(), prevent
enqueueing POSIX CPU timers on a task which has PF_EXITING set.

Queueing a timer on such a task is pointless because the task won't expire
the timer anymore.

Pretending that the timer is armed allows to keep the POSIX timer mechanism
"working" so that the timer stays accessible up to the point where a task
is unhashed.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
---
 kernel/time/posix-cpu-timers.c |   27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -628,6 +628,7 @@ static int posix_cpu_timer_del(struct k_
 	}
 
 	if (!ret) {
+		WARN_ON_ONCE(cpu_timer_queued(&timer->it.cpu));
 		put_pid(timer->it.cpu.pid);
 		timer->it_status = POSIX_TIMER_DISARMED;
 	}
@@ -674,6 +675,15 @@ void posix_cpu_timers_exit_group(struct
 	cleanup_timers(&tsk->signal->posix_cputimers);
 }
 
+static inline bool task_can_enqueue(struct k_itimer *timer, struct task_struct *p)
+{
+	if (likely(!(p->flags & PF_EXITING)))
+		return true;
+
+	/* Allow TGID type unless the last thread is on the way out. */
+	return clock_pid_type(timer->it_clock) == PIDTYPE_TGID && atomic_read(&p->signal->live);
+}
+
 /*
  * Insert the timer on the appropriate list before any timers that
  * expire later.  This must be called with the sighand lock held.
@@ -684,7 +694,24 @@ static void arm_timer(struct k_itimer *t
 	struct cpu_timer *ctmr = &timer->it.cpu;
 	u64 newexp = cpu_timer_getexpires(ctmr);
 
+	lockdep_assert_held(&p->sighand->siglock);
+
 	timer->it_status = POSIX_TIMER_ARMED;
+
+	/*
+	 * Don't enqueue timers when the task or the group is exiting. That
+	 * ensures that timer operations are still succeeding as long as the
+	 * tasks are visible, but won't enqueue the timers on the task or
+	 * process. They won't expire anyway because run_posix_cpu_timers()
+	 * exits early when PF_EXITING is set.
+	 *
+	 * Enqueue is skipped if PF_EXITING is set when the timer is per task
+	 * and when the last thread decremented p::signal::live to zero also for
+	 * per process timers.
+	 */
+	if (unlikely(!task_can_enqueue(timer, p)))
+		return;
+
 	if (!cpu_timer_enqueue(&base->tqhead, ctmr))
 		return;