Messages in this thread | | | Date | Sun, 19 Oct 2014 21:24:37 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign() |
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On 10/19, Kirill Tkhai wrote: > > 19.10.2014, 00:59, "Oleg Nesterov" <oleg@redhat.com>: > > > No, I don't think this can work. Let's look at the current code: > > > > rcu_read_lock(); > > cur = ACCESS_ONCE(dst_rq->curr); > > if (cur->pid == 0) /* idle */ > > > > And any dereference, even reading ->pid is not safe. This memory can be > > freed, unmapped, reused, etc. > > > > Looks like, task_numa_compare() needs to take dst_rq->lock and get the > > refernce first. > > Yeah, detection of idle is not save. If we reorder the checks almost all > problems will be gone. All except unmapping. JFI, is it possible with > such kernel structures as task_struct?
Yes, if DEBUG_PAGEALLOC. See kernel_map_pages() in arch/x86/mm/pageattr.c kernel_map_pages(enable => false) clears PAGE_PRESENT if slab returns the pages to system.
> --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1165,7 +1165,30 @@ 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 (cur->flags & PF_EXITING) > + cur = NULL;
so this needs probe_kernel_read(&cur->flags).
> + if (cur != ACCESS_ONCE(dst_rq->curr)) > + cur = NULL;
Yes, if this task_struct was freed in between we do not care if this memory was reused (except PF_EXITING can be false positive). If it was freed and now the same memory is ->curr again we know that delayed_put_task_struct() can't be called until we drop rcu lock, even if PF_EXITING is already set again.
I won't argue, but you need to convince Peter to accept this hack ;)
> > Or, perhaps, we need to change the rules to ensure that any "task_struct *" > > pointer is rcu-safe. Perhaps we have more similar problems... I'd like to > > avoid this if possible. > > RT tree has: > > https://git.kernel.org/cgit/linux/kernel/git/paulg/3.10-rt-patches.git/ > tree/patches/sched-delay-put-task.patch
Yes, and this obviously implies more rcu callbacks in flight, and another gp before __put_task_struct(). but may be we will need to do this anyway...
Oleg.
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