Re: [PATCH v13.3 01/14] vpci: use per-domain PCI lock to protect vpci structure

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Feb 26, 2024 at 03:47:17PM +0100, Jan Beulich wrote:
> On 21.02.2024 03:45, 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()
> > while accessing pdevs in vpci code.
> > 
> > 6. Switch vPCI functions to use per-domain pci_lock for ensuring pdevs
> > do not go away. The vPCI functions call several MSI-related functions
> > which already have existing non-vPCI callers. Change those MSI-related
> > functions to allow using either pcidevs_lock() or d->pci_lock for
> > ensuring pdevs do not go away. Holding d->pci_lock in read mode is
> > sufficient. Note that this pdev protection mechanism does not protect
> > other state or critical sections. These MSI-related functions already
> > have other race condition and state protection mechanims (e.g.
> > d->event_lock and msixtbl RCU), so we deduce that the use of the global
> > pcidevs_lock() is to ensure that pdevs do not go away.
> > 
> > 7. Introduce wrapper construct, pdev_list_is_read_locked(), for checking
> > that pdevs do not go away. The purpose of this wrapper is to aid
> > readability and document the intent of the pdev protection mechanism.
> > 
> > 8. When possible, the existing non-vPCI callers of these MSI-related
> > functions haven't been switched to use the newly introduced per-domain
> > pci_lock, and will continue to use the global pcidevs_lock(). This is
> > done to reduce the risk of the new locking scheme introducing
> > regressions. Those users will be adjusted in due time. One exception
> > is where the pcidevs_lock() in allocate_and_map_msi_pirq() is moved to
> > the caller, physdev_map_pirq(): this instance is switched to
> > read_lock(&d->pci_lock) right away.
> > 
> > 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>
> 
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> with two small remaining remarks (below) and on the assumption that an
> R-b from Roger in particular for the vPCI code is going to turn up
> eventually.
> 
> > @@ -895,6 +891,15 @@ int vpci_msix_arch_print(const struct vpci_msix *msix)
> >  {
> >      unsigned int i;
> >  
> > +    /*
> > +     * Assert that pdev_list doesn't change. 
> > ASSERT_PDEV_LIST_IS_READ_LOCKED
> > +     * is not suitable here because it may allow either pcidevs_lock() or
> > +     * pci_lock to be held, but here we rely on pci_lock being held, not
> > +     * pcidevs_lock().
> > +     */
> > +    ASSERT(rw_is_locked(&msix->pdev->domain->pci_lock));
> > +    ASSERT(spin_is_locked(&msix->pdev->vpci->lock));
> 
> As to the comment, I think it's not really "may". I also think referral to
> ...

+1 to dropping 'may'.

Thanks, Roger.