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 13:16:43 +0400 |
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And smp_rmb() beetween ifs which is pairs with rq unlocking
> 18.10.2014, 12:15, "Kirill Tkhai" <tkhai@yandex.ru>: > >> 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() ... ... > > How about this? > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index b78280c..d46427e 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1165,7 +1165,21 @@ 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; > + /* > + * Check once again to be sure curr is still on dst_rq. Even if > + * it points on a new task, which is using the memory of freed > + * cur, it's OK, because we've locked RCU before > + * delayed_put_task_struct() callback is called to put its struct. > + */ > + if (cur != ACCESS_ONCE(dst_rq->curr)) > cur = NULL; > > /* > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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