Re: [PATCH v8 1/5] mm: introduce a common interface for balloon pages mobility

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

On Tue, Aug 21, 2012 at 05:20:11PM +0200, Peter Zijlstra wrote:
> On Tue, 2012-08-21 at 09:47 -0300, Rafael Aquini wrote:
> > +       mapping = rcu_access_pointer(page->mapping);
> > +       if (mapping)
> > +               mapping = mapping->assoc_mapping; 
> 
> The comment near rcu_access_pointer() explicitly says:
> 
>  * Return the value of the specified RCU-protected pointer, but omit the
>  * smp_read_barrier_depends() and keep the ACCESS_ONCE().  This is useful
>  * when the value of this pointer is accessed, but the pointer is not
>  * dereferenced,
> 
> Yet you dereference the pointer... smells like fail to me.

Indeed!

This will break DEC Alpha.  In addition, if ->mapping can transition
from non-NULL to NULL, and if you used rcu_access_pointer() rather
than rcu_dereference() to avoid lockdep-RCU from yelling at you about
not either being in an RCU read-side critical section or holding an
update-side lock, you can see failures as follows:

1.	CPU 0 runs the above code, picks up mapping, and finds it non-NULL.

2.	CPU 0 is preempted or otherwise delayed.  (Keep in mind that
	even disabling interrupts in a guest OS does not prevent the
	host hypervisor from preempting!)

3.	Some other CPU NULLs page->mapping.  Because CPU 0 isn't doing
	anything to prevent it, this other CPU frees the memory.

4.	CPU 0 resumes, and then accesses what is now the freelist.
	Arbitrarily bad things start happening.

If you are in a read-side critical section, use rcu_dereference() instead
of rcu_access_pointer().  If you are holding an update-side lock, use
rcu_dereference_protected() and say what lock you are holding.  If you
are doing something else, please say what it is.

							Thanx, Paul

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