Re: [RFC PATCH v2 02/11] PCI/P2PDMA: Avoid pci_get_slot() which sleeps

From: Bjorn Helgaas <helgaas@kernel.org>
To: Logan Gunthorpe <logang@deltatee.com>
Cc: 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,
	"Stephen Bates" <sbates@raithlin.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Christian König" <christian.koenig@amd.com>,
	"Ira Weiny" <iweiny@intel.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Don Dutile" <ddutile@redhat.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>
Subject: Re: [RFC PATCH v2 02/11] PCI/P2PDMA: Avoid pci_get_slot() which sleeps
Date: Fri, 12 Mar 2021 14:57:07 -0600	[thread overview]
Message-ID: <20210312205707.GA2288658@bjorn-Precision-5520> (raw)
In-Reply-To: <20210311233142.7900-3-logang@deltatee.com>

On Thu, Mar 11, 2021 at 04:31:32PM -0700, 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
> list. It should be impossible for the host bridges device to go away
> while references are held on child devices, so the first element
> should not change and this should be safe.
> 
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> ---
>  drivers/pci/p2pdma.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index bd89437faf06..2135fe69bb07 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -311,11 +311,15 @@ 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;
>  
>  	if (!root)
> +		root = list_first_entry_or_null(&host->bus->devices,
> +						struct pci_dev, bus_list);

Replacing one ugliness (assuming there is a pci_dev for the host
bridge, and that it is at 00.0) with another (still assuming a pci_dev
and that it is host->bus->self or the first entry).  I can't suggest
anything better, but maybe a little comment in the code would help
future readers.

I wish we had a real way to discover this property without the
whitelist, at least for future devices.  Was there ever any interest
in a _DSM or similar interface for this?

I *am* very glad to remove a pci_get_slot() usage.

> +
> +	if (!root || root->devfn)
>  		return false;
>  
>  	vendor = root->vendor;

Don't you need to also remove the "pci_dev_put(root)" a few lines
below?

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