On Fri, Mar 29, 2019 at 10:50 AM Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote:
>
> Thanks Dan, this is great. I think the changes in this series are
> cleaner and more understandable than the patch set I had sent earlier.
>
> However, I found a couple minor issues with this patch:
>
> On 2019-03-29 9:27 a.m., Dan Williams wrote:
> > static void pci_p2pdma_release(void *data)
> > {
> > struct pci_dev *pdev = data;
> > @@ -103,12 +110,12 @@ static void pci_p2pdma_release(void *data)
> > if (!pdev->p2pdma)
> > return;
> >
> > - wait_for_completion(&pdev->p2pdma->devmap_ref_done);
> > - percpu_ref_exit(&pdev->p2pdma->devmap_ref);
> > + /* Flush and disable pci_alloc_p2p_mem() */
> > + pdev->p2pdma = NULL;
> > + synchronize_rcu();
> >
> > gen_pool_destroy(pdev->p2pdma->pool);
>
> I missed this on my initial review, but it became obvious when I tried
> to test the series: this is a NULL dereference seeing pdev->p2pdma was
> set to NULL a few lines up.
Ah, yup.
> When I fix this by storing p2pdma in a local variable, the patch set
> works and never seems to crash when I hot remove p2pdma memory.
Great!
>
> > void *pci_alloc_p2pmem(struct pci_dev *pdev, size_t size)
> > {
> > - void *ret;
> > + void *ret = NULL;
> > + struct percpu_ref *ref;
> >
> > + rcu_read_lock();
> > if (unlikely(!pdev->p2pdma))
> > - return NULL;
>
> Using RCU here makes sense to me, however I expect we should be using
> the proper rcu_assign_pointer(), rcu_dereference() and __rcu tag with
> pdev->p2pdma. If only to better document what's being protected with the
> new RCU calls.
I think just add a comment because those helpers are for cases where
the rcu protected pointer is allowed to race the teardown. In this
case we're using rcu just as a barrier to force the NULL check to
resolve.
