linux-kernel - Re: [PATCH 5/6] pci/p2pdma: Track pgmap references per resource, not globally

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Date:   Fri, 29 Mar 2019 11:50:23 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Dan Williams <dan.j.williams@...el.com>, akpm@...ux-foundation.org
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Christoph Hellwig <hch@....de>, linux-mm@...ck.org,
        linux-pci@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] pci/p2pdma: Track pgmap references per resource, not
 globally

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.

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.

>  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.

Logan