Re: [PATCH 02/16] PCI/P2PDMA: Avoid pci_get_slot() which sleeps

From: Logan Gunthorpe <logang@deltatee.com>
To: Don Dutile <ddutile@redhat.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>,
	"Christian König" <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>
Subject: Re: [PATCH 02/16] PCI/P2PDMA: Avoid pci_get_slot() which sleeps
Date: Tue, 11 May 2021 10:14:37 -0600	[thread overview]
Message-ID: <70f58fff-c228-e637-aef8-f404af2d2f9d@deltatee.com> (raw)
In-Reply-To: <09f6c713-5ae6-035c-c0ab-a8bd0fac7a79@redhat.com>

On 2021-05-11 10:05 a.m., Don Dutile wrote:
> 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?

I've dropped the bus->self thing in v2. Seems bus->self is explicitly
unset for root bridges. There's remnants in the pci code that used to
check bus->self to see if the bridge is the root bridge.

I tried setting bus->self with the pci device of the root bridge but
that just caused my machine not to boot and I didn't dig any further.

Logan