Re: kmemcheck caught read from freed memory (cfq_free_io_context)

[Jens Axboe <jens.axboe@oracle.com>]

On Wed, Apr 02 2008, Peter Zijlstra wrote:
> On Wed, 2008-04-02 at 13:07 +0200, Jens Axboe wrote:
> > On Wed, Apr 02 2008, Pekka Enberg wrote:
> > > Hi Paul,
> > > 
> > > On Wed, Apr 2, 2008 at 1:55 PM, Paul E. McKenney
> > > <paulmck@linux.vnet.ibm.com> wrote:
> > > >  I will check this when I get back to some bandwidth -- but in the meantime,
> > > >  does kmemcheck special-case SLAB_DESTROY_BY_RCU?  It is legal to access
> > > >  newly-freed items in that case, as long as you did rcu_read_lock()
> > > >  before gaining a reference to them and don't hold the reference past
> > > >  the matching rcu_read_unlock().
> > > 
> > > No, kmemcheck is work in progress and does not know about
> > > SLAB_DESTROY_BY_RCU yet. The reason I asked Vegard to post the warning
> > > was because Peter, Vegard, and myself identified this particular
> > > warning as a real problem. But yeah, kmemcheck can cause false
> > > positives for RCU for now.
> > 
> > Makes sense, and to me Pauls analysis of the code looks totally correct
> > - there's no bug there, at least related to hlist traversal and
> > kmem_cache_free(), since we are under rcu_read_lock() and thus hold off
> > the grace for freeing.
> 
> but what holds off the slab allocator re-issueing that same object and
> someone else writing other stuff into it?

Nothing, that's how rcu destry works here. But for the validation to be
WRONG radix_tree_lookup(..., old_key) must return cic for new_key, not
NULL.

-- 
Jens Axboe