On Mon, Aug 25, 2014 at 05:57:38PM +0200, Oleg Nesterov wrote:
> Peter, do you remember another problem with TASK_DEAD we discussed recently?
> (prev_state == TASK_DEAD detection in finish_task_switch() still looks racy).
Uhm, right. That was somewhere on the todo list :-)
> I am starting to think that perhaps we need something like below, what do
> you all think?
I'm thinking you lost the hunk that adds rq::dead :-), more comments
below.
> --- x/kernel/sched/core.c
> +++ x/kernel/sched/core.c
> @@ -2205,9 +2205,10 @@ static void finish_task_switch(struct rq
> __releases(rq->lock)
> {
> struct mm_struct *mm = rq->prev_mm;
> - long prev_state;
> + struct task_struct *dead = rq->dead;
>
> rq->prev_mm = NULL;
> + rq->dead = NULL;
>
> /*
> * A task struct has one reference for the use as "current".
> @@ -2220,7 +2221,6 @@ static void finish_task_switch(struct rq
> * be dropped twice.
> * Manfred Spraul <manf...@colorfullife.com>
> */
Clearly that comment needs to go as well..
> - prev_state = prev->state;
> vtime_task_switch(prev);
> finish_arch_switch(prev);
> perf_event_task_sched_in(prev, current);
> @@ -2230,16 +2230,16 @@ static void finish_task_switch(struct rq
> fire_sched_in_preempt_notifiers(current);
> if (mm)
> mmdrop(mm);
> - if (unlikely(prev_state == TASK_DEAD)) {
> - if (prev->sched_class->task_dead)
> - prev->sched_class->task_dead(prev);
> + if (unlikely(dead)) {
BUG_ON(dead != prev); ?
> + if (dead->sched_class->task_dead)
> + dead->sched_class->task_dead(dead);
>
> /*
> * Remove function-return probe instances associated with this
> * task and put them back on the free list.
> */
> - kprobe_flush_task(prev);
> - put_task_struct(prev);
> + kprobe_flush_task(dead);
> + put_task_struct(dead);
> }
>
> tick_nohz_task_switch(current);
> @@ -2770,11 +2770,15 @@ need_resched:
> smp_mb__before_spinlock();
> raw_spin_lock_irq(&rq->lock);
>
> + if (unlikely(rq->dead))
> + goto deactivate;
> +
Yeah, it would be best to not have to do that; ideally we would be able
to maybe do both; set rq->dead and current->state == TASK_DEAD.
Hmm, your exit_schedule() already does this, so why this extra test?
> switch_count = &prev->nivcsw;
> if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
> if (unlikely(signal_pending_state(prev->state, prev))) {
> prev->state = TASK_RUNNING;
> } else {
> +deactivate:
> deactivate_task(rq, prev, DEQUEUE_SLEEP);
> prev->on_rq = 0;
>
> @@ -2826,6 +2830,15 @@ need_resched:
> goto need_resched;
> }
>
> +// called under preempt_disable();
> +void exit_schedule()
> +{
> + // TODO: kill TASK_DEAD, this is only for proc
> + current->state = TASK_DEAD;
Ah, not so, its also to avoid that extra condition in schedule() :-)
> + task_rq(current)->dead = current;
> + __schedule();
> +}
> +
> static inline void sched_submit_work(struct task_struct *tsk)
> {
> if (!tsk->state || tsk_is_pi_blocked(tsk))
> --- x/kernel/exit.c
> +++ x/kernel/exit.c
> @@ -815,25 +815,8 @@ void do_exit(long code)
> __this_cpu_add(dirty_throttle_leaks, tsk->nr_dirtied);
> exit_rcu();
>
> - /*
> - * The setting of TASK_RUNNING by try_to_wake_up() may be delayed
> - * when the following two conditions become true.
> - * - There is race condition of mmap_sem (It is acquired by
> - * exit_mm()), and
> - * - SMI occurs before setting TASK_RUNINNG.
> - * (or hypervisor of virtual machine switches to other guest)
> - * As a result, we may become TASK_RUNNING after becoming TASK_DEAD
> - *
> - * To avoid it, we have to wait for releasing tsk->pi_lock which
> - * is held by try_to_wake_up()
> - */
> - smp_mb();
> - raw_spin_unlock_wait(&tsk->pi_lock);
> -
> - /* causes final put_task_struct in finish_task_switch(). */
> - tsk->state = TASK_DEAD;
> tsk->flags |= PF_NOFREEZE; /* tell freezer to ignore us */
> - schedule();
> + exit_schedule();
> BUG();
> /* Avoid "noreturn function does return". */
> for (;;)
Yes, something like this might work fine..
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