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Re: [RFC PATCH 08/10] xen: pci: remove pcidev_[un]lock[ed] calls
- To: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 28 Feb 2023 17:51:34 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Paul Durrant <paul@xxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>
- Delivery-date: Tue, 28 Feb 2023 16:52:20 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 28.01.2023 02:32, Stefano Stabellini wrote:
> On Wed, 31 Aug 2022, Volodymyr Babchuk wrote:
>> As pci devices are refcounted now and all list that store them are
>> protected by separate locks, we can safely drop global pcidevs_lock.
>>
>> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
>
> Up until this patch this patch series introduces:
> - d->pdevs_lock to protect d->pdev_list
> - pci_seg->alldevs_lock to protect pci_seg->alldevs_list
> - iommu->ats_list_lock to protect iommu->ats_devices
> - pdev refcounting to detect a pdev in-use and when to free it
> - pdev->lock to protect pdev->msi_list
>
> They cover a lot of ground. Are they collectively covering everything
> pcidevs_lock() was protecting?
>
> deassign_device is not protected by pcidevs_lock anymore.
> deassign_device accesses a number of pdev fields, including quarantine,
> phantom_stride and fault.count.
>
> deassign_device could run at the same time as assign_device who sets
> quarantine and other fields.
>
> It looks like assign_device, deassign_device, and other functions
> accessing/modifying pdev fields should be protected by pdev->lock.
>
> In fact, I think it would be safer to make sure every place that
> currently has a pcidevs_lock() gets a pdev->lock (unless there is a
> d->pdevs_lock, pci_seg->alldevs_lock, iommu->ats_list_lock, or another
> lock) ?
Yes, I agree - there shouldn't be cases where lock uses are removed
with neither replacement nor an explanation why the removal is safe.
Which in turn suggests that a change like the one here likely needs
doing in much smaller chunks. Grouping could possibly be based upon
all touched instances having the same replacement or justification.
Jan
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