Messages in this thread | | | From | Kirill Tkhai <> | Subject | Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign() | Date | Sat, 18 Oct 2014 12:15:01 +0400 |
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18.10.2014, 01:40, "Oleg Nesterov" <oleg@redhat.com>: > The lockless get_task_struct(tsk) is only safe if tsk == current > and didn't pass exit_notify(), or if this tsk was found on a rcu > protected list (say, for_each_process() or find_task_by_vpid()). > IOW, it is only safe if release_task() was not called before we > take rcu_read_lock(), in this case we can rely on the fact that > delayed_put_pid() can not drop the (potentially) last reference > until rcu_read_unlock(). > > And as Kirill pointed out task_numa_compare()->task_numa_assign() > path does get_task_struct(dst_rq->curr) and this is not safe. The > task_struct itself can't go away, but rcu_read_lock() can't save > us from the final put_task_struct() in finish_task_switch(); this > reference goes away without rcu gp. > > Reported-by: Kirill Tkhai <ktkhai@parallels.com> > Signed-off-by: Oleg Nesterov <oleg@redhat.com> > --- > kernel/sched/fair.c | 8 +++++++- > 1 files changed, 7 insertions(+), 1 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 0090e8c..52049b9 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1158,7 +1158,13 @@ static void task_numa_compare(struct task_numa_env *env, > > rcu_read_lock(); > cur = ACCESS_ONCE(dst_rq->curr); > - if (cur->pid == 0) /* idle */ > + /* > + * No need to move the exiting task, and this ensures that ->curr > + * wasn't reaped and thus get_task_struct() in task_numa_assign() > + * is safe; note that rcu_read_lock() can't protect from the final > + * put_task_struct() after the last schedule(). > + */ > + if (is_idle_task(cur) || (cur->flags & PF_EXITING)) > cur = NULL; > > /*
Oleg, I've looked once again, and now it's not good for me. Where is the guarantee this memory hasn't been allocated again? If so, PF_EXITING is not of the task we are interesting, but it's not a task's even.
rcu_read_lock() ... ... cur = ACCESS_ONCE(dst_rq->curr); ... ... <interrupt> rq->curr = next; ... <interrupt> put_prev_task() ... <interrupt> __put_prev_task ... <interrupt> kmem_cache_free() ... <interrupt> ... <alocated again> <interrupt> ... memset(, 0, ) <interrupt> ... ... if (cur->flags & PF_EXITING) ... ... <no> ... ... get_task_struct() ... ...
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