linux-kernel - Re: [PATCH 02/16] PCI/P2PDMA: Avoid pci_get

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Message-ID: <70f58fff-c228-e637-aef8-f404af2d2f9d@deltatee.com>
Date:   Tue, 11 May 2021 10:14:37 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Don Dutile <ddutile@...hat.com>, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-mm@...ck.org,
        iommu@...ts.linux-foundation.org
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 02/16] PCI/P2PDMA: Avoid pci_get_slot() which sleeps



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