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    SubjectRe: [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.

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