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Subject: Re: [PATCH 02/16] PCI/P2PDMA: Avoid pci_get_slot() which sleeps
To:     Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-mm@kvack.org, iommu@lists.linux-foundation.org
Cc:     Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Matthew Wilcox <willy@infradead.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Jakowski Andrzej <andrzej.jakowski@intel.com>,
        Minturn Dave B <dave.b.minturn@intel.com>,
        Jason Ekstrand <jason@jlekstrand.net>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Xiong Jianxin <jianxin.xiong@intel.com>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Robin Murphy <robin.murphy@arm.com>
References: <20210408170123.8788-1-logang@deltatee.com>
 <20210408170123.8788-3-logang@deltatee.com>
From:   Don Dutile <ddutile@redhat.com>
Message-ID: <09f6c713-5ae6-035c-c0ab-a8bd0fac7a79@redhat.com>
Date:   Tue, 11 May 2021 12:05:34 -0400
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On 4/8/21 1:01 PM, Logan Gunthorpe wrote:
> In order to use upstream_bridge_distance_warn() from a dma_map function,
> it must not sleep. However, pci_get_slot() takes the pci_bus_sem so it
> might sleep.
>
> In order to avoid this, try to get the host bridge's device from
> bus->self, and if that is not set, just get the first element in the
> device list. It should be impossible for the host bridge's device to
> go away while references are held on child devices, so the first element
> should not be able to change and, thus, this should be safe.
Bjorn:
Why wouldn't (shouldn't?) the bus->self field be set for a host bridge device?
Should this situation be repaired in the host-brige config/setup code elsewhere in the kernel.
... and here, a check-and-fail with info of what doesn't have it setup (another new pci function to do the check & prinfo), so it can point to the offending host-bridge, and thus, the code that needs to be updated?


> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> ---
>   drivers/pci/p2pdma.c | 14 ++++++++++++--
>   1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index bd89437faf06..473a08940fbc 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -311,16 +311,26 @@ static const struct pci_p2pdma_whitelist_entry {
>   static bool __host_bridge_whitelist(struct pci_host_bridge *host,
>   				    bool same_host_bridge)
>   {
> -	struct pci_dev *root = pci_get_slot(host->bus, PCI_DEVFN(0, 0));
>   	const struct pci_p2pdma_whitelist_entry *entry;
> +	struct pci_dev *root = host->bus->self;
>   	unsigned short vendor, device;
>   
> +	/*
> +	 * This makes the assumption that the first device on the bus is the
> +	 * bridge itself and it has the devfn of 00.0. This assumption should
> +	 * hold for the devices in the white list above, and if there are cases
> +	 * where this isn't true they will have to be dealt with when such a
> +	 * case is added to the whitelist.
> +	 */
>   	if (!root)
> +		root = list_first_entry_or_null(&host->bus->devices,
> +						struct pci_dev, bus_list);
> +
> +	if (!root || root->devfn)
>   		return false;
>   
>   	vendor = root->vendor;
>   	device = root->device;
> -	pci_dev_put(root);
>   
>   	for (entry = pci_p2pdma_whitelist; entry->vendor; entry++) {
>   		if (vendor != entry->vendor || device != entry->device)