Re: Is it OK to pass non-acquired objects to kfree?

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Sep 10, 2015 at 08:26:59PM +0200, Dmitry Vyukov wrote:
> On Thu, Sep 10, 2015 at 8:13 PM, Christoph Lameter <cl@linux.com> wrote:
> > On Thu, 10 Sep 2015, Dmitry Vyukov wrote:
> >
> >> On Thu, Sep 10, 2015 at 8:01 PM, Christoph Lameter <cl@linux.com> wrote:
> >> > On Thu, 10 Sep 2015, Paul E. McKenney wrote:
> >> >
> >> >> The reason we poked at this was to see if any of SLxB touched the
> >> >> memory being freed.  If none of them touched the memory being freed,
> >> >> and if that was a policy, then the idiom above would be legal.  However,
> >> >> one of them does touch the memory being freed, so, yes, the above code
> >> >> needs to be fixed.
> >> >
> >> > The one that touches the object has a barrier() before it touches the
> >> > memory.
> >>
> >> It does not change anything, right?
> >
> > It changes the first word of the object after the barrier. The first word
> > is used in SLUB as the pointer to the next free object.
> 
> User can also write to this object after it is reallocated. It is
> equivalent to kmalloc writing to the object.
> And barrier is not the kind of barrier that would make it correct.
> So I do not see how it is relevant.

I believe that the two of you are talking past each other.  It sounds
to me that Christoph is arguing that SL*B is correctly implemented,
and that Dmitry is arguing that the use case is broken.

>From what I can see, both are correct.

							Thanx, Paul

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