Messages in this thread Patch in this message | | | From | Kirill Tkhai <> | Subject | Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign() | Date | Sat, 18 Oct 2014 12:33:27 +0400 |
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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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