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Re: [PATCH v2] vpci: introduce per-domain lock to protect vpci structure
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>
- Date: Tue, 15 Feb 2022 10:57:18 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, "jbeulich@xxxxxxxx" <jbeulich@xxxxxxxx>, "julien@xxxxxxx" <julien@xxxxxxx>, "sstabellini@xxxxxxxxxx" <sstabellini@xxxxxxxxxx>, Oleksandr Tyshchenko <Oleksandr_Tyshchenko@xxxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Artem Mygaiev <Artem_Mygaiev@xxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Rahul Singh <rahul.singh@xxxxxxx>, Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>
- Delivery-date: Tue, 15 Feb 2022 10:57:32 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHYIkOs9pmsZb0l5kefj1m3qdTVCayUboeAgAACiYA=
- Thread-topic: [PATCH v2] vpci: introduce per-domain lock to protect vpci structure
On 15.02.22 12:48, Roger Pau Monné wrote:
> On Tue, Feb 15, 2022 at 10:11:35AM +0200, Oleksandr Andrushchenko wrote:
> @@ -911,7 +914,11 @@ int vpci_msix_arch_print(const struct vpci_msix *msix)
>> struct pci_dev *pdev = msix->pdev;
>>
>> spin_unlock(&msix->pdev->vpci->lock);
>> + pcidevs_unlock();
>> + read_unlock(&pdev->domain->vpci_rwlock);
>> process_pending_softirqs();
>> + read_lock(&pdev->domain->vpci_rwlock);
>> + pcidevs_lock();
> This is again an ABBA situation: vpci_add_handlers will get called
> with pci_devs locked, and it will try to acquire the per-domain vpci
> lock (so pcidevs -> vpci_rwlock) while here and in other places in the
> patch to you have inverse locking order (vpci_rwlock -> pcidevs).
Indeed, I need to always lock in this order: pcidevs -> vpci_rwlock
to prevent ABBA, good catch
>
>> /* NB: we assume that pdev cannot go away for an alive domain.
>> */
>> if ( !pdev->vpci || !spin_trylock(&pdev->vpci->lock) )
>> return -EBUSY;
>> @@ -323,10 +334,18 @@ uint32_t vpci_read(pci_sbdf_t sbdf, unsigned int reg,
>> unsigned int size)
>> }
>>
>> /* Find the PCI dev matching the address. */
>> + pcidevs_lock();
>> pdev = pci_get_pdev_by_domain(d, sbdf.seg, sbdf.bus, sbdf.devfn);
>> + pcidevs_unlock();
>> if ( !pdev )
>> return vpci_read_hw(sbdf, reg, size);
> There's a window here (between dropping the pcidevs lock and acquiring
> the vpci_rwlock where either the pdev or pdev->vpci could be removed
> or recreated.
Yes, I know that. But this is the best I came up with...
>
> Thanks, Roger.
Thank you,
Oleksandr
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