Re: [PATCH v12.1 01/15] vpci: use per-domain PCI lock to protect vpci structure

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.01.2024 19:14, Stewart Hildebrand wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> 
> Use the per-domain PCI read/write lock to protect the presence of the
> pci device vpci field. This lock can be used (and in a few cases is used
> right away) so that vpci removal can be performed while holding the lock
> in write mode. Previously such removal could race with vpci_read for
> example.
> 
> When taking both d->pci_lock and pdev->vpci->lock, they should be
> taken in this exact order: d->pci_lock then pdev->vpci->lock to avoid
> possible deadlock situations.
> 
> 1. Per-domain's pci_lock is used to protect pdev->vpci structure
> from being removed.
> 
> 2. Writing the command register and ROM BAR register may trigger
> modify_bars to run, which in turn may access multiple pdevs while
> checking for the existing BAR's overlap. The overlapping check, if
> done under the read lock, requires vpci->lock to be acquired on both
> devices being compared, which may produce a deadlock. It is not
> possible to upgrade read lock to write lock in such a case. So, in
> order to prevent the deadlock, use d->pci_lock in write mode instead.
> 
> All other code, which doesn't lead to pdev->vpci destruction and does
> not access multiple pdevs at the same time, can still use a
> combination of the read lock and pdev->vpci->lock.
> 
> 3. Drop const qualifier where the new rwlock is used and this is
> appropriate.
> 
> 4. Do not call process_pending_softirqs with any locks held. For that
> unlock prior the call and re-acquire the locks after. After
> re-acquiring the lock there is no need to check if pdev->vpci exists:
>  - in apply_map because of the context it is called (no race condition
>    possible)
>  - for MSI/MSI-X debug code because it is called at the end of
>    pdev->vpci access and no further access to pdev->vpci is made
> 
> 5. Use d->pci_lock around for_each_pdev and pci_get_pdev_by_domain
> while accessing pdevs in vpci code.
> 
> Suggested-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
> Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

While I know Roger did offer the tag with certain adjustments, ...

> @@ -913,7 +911,12 @@ int vpci_msix_arch_print(const struct vpci_msix *msix)
>              struct pci_dev *pdev = msix->pdev;
>  
>              spin_unlock(&msix->pdev->vpci->lock);
> +            read_unlock(&pdev->domain->pci_lock);
>              process_pending_softirqs();
> +
> +            if ( !read_trylock(&pdev->domain->pci_lock) )
> +                return -EBUSY;
> +
>              /* NB: we assume that pdev cannot go away for an alive domain. */
>              if ( !pdev->vpci || !spin_trylock(&pdev->vpci->lock) )
>                  return -EBUSY;

... I'm sure he was assuming you would get this right, in also
dropping the 1st-try-acquired lock when this 2nd try-lock fails.
Personally I feel this is the kind of change one would better not
offer (or take) R-b ahead of time.

I further think the respective comment in vpci_dump_msi() also wants
adjusting from singular to plural.

Jan