linux-kernel - Re: [PATCH] PCI/P2PDMA: Annotate RCU dereference

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Date:   Tue, 14 Feb 2023 16:08:49 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Chaitanya Kulkarni <chaitanyak@...dia.com>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] PCI/P2PDMA: Annotate RCU dereference

On Thu, Feb 09, 2023 at 10:29:53AM -0700, Logan Gunthorpe wrote:
> A dereference of the __rcu pointer was noticed by sparse:
> 
>   drivers/pci/p2pdma.c:199:44: sparse: sparse: dereference of noderef expression
> 
> The __rcu pointer should be dereferenced using
> rcu_dereference_protected() instead of accessed directly. Its safe
> to use rcu_derference_protected() seeing a reference is held on
> the pgmap's percpu reference counter and thus it cannot disappear.
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>

Applied with Chaitanya's reviewed-by to pci/p2pdma for v6.3, thanks!

> ---
> 
> Based on v6.2-rc7
> 
>  drivers/pci/p2pdma.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 86812d2073ea..9e8205572830 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -194,11 +194,13 @@ static const struct attribute_group p2pmem_group = {
>  static void p2pdma_page_free(struct page *page)
>  {
>  	struct pci_p2pdma_pagemap *pgmap = to_p2p_pgmap(page->pgmap);
> +	/* safe to dereference while a reference is held to the percpu ref */
> +	struct pci_p2pdma *p2pdma =
> +		rcu_dereference_protected(pgmap->provider->p2pdma, 1);
>  	struct percpu_ref *ref;
> 
> -	gen_pool_free_owner(pgmap->provider->p2pdma->pool,
> -			    (uintptr_t)page_to_virt(page), PAGE_SIZE,
> -			    (void **)&ref);
> +	gen_pool_free_owner(p2pdma->pool, (uintptr_t)page_to_virt(page),
> +			    PAGE_SIZE, (void **)&ref);
>  	percpu_ref_put(ref);
>  }
> 
> 
> base-commit: 4ec5183ec48656cec489c49f989c508b68b518e3
> --
> 2.30.2