Messages in this thread | | | Date | Sun, 19 Oct 2014 21:37:44 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign() |
| |
On 10/19, Oleg Nesterov wrote: > > On 10/19, Kirill Tkhai wrote: > > > > --- 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...
Forgot to mention... Or we can make task_struct_cachep SLAB_DESTROY_BY_RCU, in this case ->curr (or any other "task_struct *" ponter) can not go away under rcu_read_lock(). task_numa_compare() still needs the PF_EXITING check, but we do not need to recheck ->curr or probe_kernel_read().
Oleg.
-- 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/
| |