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SubjectRe: kmemcheck caught read from freed memory (cfq_free_io_context)
Hi Paul,

On Wed, Apr 02, 2008 at 07:32:26PM +0300, Pekka J Enberg wrote:
> > Well, maybe we can add two new states: RCU_FREED and RCU_VALIDATED? The
> > object is flagged with the first one as soon as an object is handed over
> > to kmem_cache_free() and the latter needs to hook to the validation phase
> > of RCU (how is that done btw?). Then kmemcheck could even give a better
> > error message: "RCU-freed object used without validation."
> >
> > And with delayed free for kmemcheck we discussed before, we'd hold on to
> > the objects long enough to actually see these error conditions.

On Wed, Apr 2, 2008 at 9:23 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> Well, one approach would be to add an rcu_head to the kmem_cache
> structure, along with a flag stating that the rcu_head is in use. I hope
> that there is a better approach, as this introduces a lock roundtrip
> into kmemcheck_slab_free(). Is there a better place to put the rcu_head?
> Perhaps into the per-CPU allocator? But then we have to track which
> CPU has which mark pending, and there are only so many bits in a byte,
> as the SGI guys would be quick to point out

I suppose you haven't actually run kmemcheck on your machine? We're
taking a page fault for _every_ memory access so a lock round-trip in
the SLAB_RCU case is probably not that bad performance-wise :-).


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