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

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Date:   Tue, 11 May 2021 10:16:40 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Don Dutile <ddutile@...hat.com>,
        John Hubbard <jhubbard@...dia.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>,
        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:
> ... add a flag (set for p2pdma use)  to the function to print out what the root->devfn is, and what
> the device is so the needed quirk &/or modification can added to handle when this assumption fails;
> or make it a prdebug that can be flipped on for this failing situation, again, to add needed change to accomodate.

Good idea! Will add.

>>     root = NULL;
>>  out:
>>     pci_dev_get(root);
>>     return root;
>> }
>> EXPORT_SYMBOL(pci_get_root_slot);
>>
>> ...I think that's a lot clearer to the reader, about what's going on here.
>>
>> Note that I'm not really sure if it *is* safe, I would need to ask other
>> PCIe subsystem developers with more experience. But I don't think anyone
>> is trying to make p2pdma calls so early that PCIe buses are uninitialized.
>>
>>
>>> +
>>> +    if (!root || root->devfn)
>>>           return false;
>>>         vendor = root->vendor;
>>>       device = root->device;
>>> -    pci_dev_put(root);
> and the reason to remove the dev_put is b/c it can sleep as well?
> is that ok, given the dev_get that John put into the new pci_get_root_slot()?
> ... seems like a locking version with no get/put's is needed, or, fix the host-bridge setups so no !NULL self pointers.

The dev_get is redundant here seeing we hold references to child
devices. It was only in the previous code because we were using
pci_get_slot() to get the device which did the get for us.

Logan