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SubjectRe: [PATCH v3] sched/numa: fix unsafe get_task_struct() in task_numa_assign()
On 10/21, Peter Zijlstra wrote:
> On Mon, Oct 20, 2014 at 10:50:06PM +0200, Oleg Nesterov wrote:
> > Let me explain what I personally dislike in v3:
> >
> > - I think that we do not have enough reasons for
> > SLAB_DESTROY_BY_RCU. This is the serious change.
> What exactly would the downsides be? SDBR has very limited space
> overhead iirc.

Yes, SDBR is nice (and it could probably have more users), but my
concern is not overhead. Please see below.

> > - Again, perhaps we should start we a simple and stupid fix.
> > We can do get_task_struct() under rq->lock or, if nothing
> > else, just
> >
> > raw_spin_lock_irq(&rq->lock);
> > cur = rq->curr;
> > if (is_idle_task(cur) || (cur->flags & PF_EXITING))
> > cur = NULL;
> > raw_spin_unlock_irq(&rq->lock);
> I think I agree with you, this is the simple safe option and is
> something we can easily backport. After that we can add creative bits on
> top.


Kirill, could you please make a patch?

> I think I prefer the SLAB_DESTROY_BY_RCU thing over the probe_kernel
> thing

I won't really insist, but let me try to explain why I dislike it in
this particular case.

- It is not clear who else (except task_numa_compare) will need it.
And it looks at bit strange to make task_struct SLAB_DESTROY_BY_RCU
just to read a word in task_numa_compare().

- In some sense, the usage of SDBR looks simply "wrong" in this case.
IOW, I agree that probe_kernel_read() is ugly, but in this case
SDBR acts exactly the same way as probe_kernel_read().

SDBR does not make the object rcu-safe, it only protects the object
type plus ensures that this memory can't go away. It was designed
for the case when you read the stable members initialized in ctor
(usually a lock) and verify/lock the object.

But in this case we can not detect that the object is still alive
without the additional trick, so when you read ->sighand or ->flags,
the fact that this memory is still "struct task_struct" doesn't help
and doesn't matter at all. Only the subsequent "task == rq->curr"
check proves that yes, this is task_struct.

OTOH, (afaics) we only need probe_kernel_read() if CONFIG_DEBUG_SLAB.
And in fact I think that "read the valid but potentially freed kernel
pointer" deserves another helper, it can have more users. For example,
please look at get_freepointer_safe().

What if we add get_kernel(x, ptr) macro to factor out the IS_ENABLED()
of ifdef hack? Or inline function... This way the new xxx() helper we
discussed won't look that bad.

But again, I agree that this subjective, I won't really argue.


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