[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v12.2 01/15] vpci: use per-domain PCI lock to protect vpci structure
On 25.01.2024 10:05, Roger Pau Monné wrote: > On Thu, Jan 25, 2024 at 08:43:05AM +0100, Jan Beulich wrote: >> On 24.01.2024 18:51, Roger Pau Monné wrote: >>> On Wed, Jan 24, 2024 at 12:34:10PM +0100, Jan Beulich wrote: >>>> On 24.01.2024 10:24, Roger Pau Monné wrote: >>>>> On Wed, Jan 24, 2024 at 09:48:35AM +0100, Jan Beulich wrote: >>>>>> On 23.01.2024 16:07, Roger Pau Monné wrote: >>>>>>> On Tue, Jan 23, 2024 at 03:32:12PM +0100, Jan Beulich wrote: >>>>>>>> On 15.01.2024 20:43, Stewart Hildebrand wrote: >>>>>>>>> @@ -2888,6 +2888,8 @@ int allocate_and_map_msi_pirq(struct domain *d, >>>>>>>>> int index, int *pirq_p, >>>>>>>>> { >>>>>>>>> int irq, pirq, ret; >>>>>>>>> >>>>>>>>> + ASSERT(pcidevs_locked() || rw_is_locked(&d->pci_lock)); >>>>>>>> >>>>>>>> If either lock is sufficient to hold here, ... >>>>>>>> >>>>>>>>> --- a/xen/arch/x86/physdev.c >>>>>>>>> +++ b/xen/arch/x86/physdev.c >>>>>>>>> @@ -123,7 +123,9 @@ int physdev_map_pirq(domid_t domid, int type, int >>>>>>>>> *index, int *pirq_p, >>>>>>>>> >>>>>>>>> case MAP_PIRQ_TYPE_MSI: >>>>>>>>> case MAP_PIRQ_TYPE_MULTI_MSI: >>>>>>>>> + pcidevs_lock(); >>>>>>>>> ret = allocate_and_map_msi_pirq(d, *index, pirq_p, type, >>>>>>>>> msi); >>>>>>>>> + pcidevs_unlock(); >>>>>>>>> break; >>>>>>>> >>>>>>> >>>>>>> IIRC (Stewart can further comment) this is done holding the pcidevs >>>>>>> lock to keep the path unmodified, as there's no need to hold the >>>>>>> per-domain rwlock. >>>>>> >>>>>> Yet why would we prefer to acquire a global lock when a per-domain one >>>>>> suffices? >>>>> >>>>> I was hoping to introduce less changes, specially if they are not >>>>> strictly required, as it's less risk. I'm always quite worry of >>>>> locking changes. >>>> >>>> In which case more description / code commenting is needed. The pattern >>>> of the assertions looks dangerous. >>> >>> Is such dangerousness perception because you fear some of the pcidevs >>> lock usage might be there not just for preventing the pdev from going >>> away, but also to guarantee exclusive access to certain state? >> >> Indeed. In my view the main purpose of locks is to guard state. Their >> use here to guard against devices here is imo rather an abuse; as >> mentioned before this should instead be achieved e.g via refcounting. >> And it's bad enough already that pcidevs_lock() alone has been abused >> this way, without proper marking (leaving us to guess in many places). >> It gets worse when a second lock can now also serve this same >> purpose. > > The new lock is taken in read mode in most contexts, and hence can't > be used to indirectly gain exclusive access to domain related > structures in a safe way. Oh, right - I keep being misled by rw_is_locked(). This is a fair argument. Irrespective it would feel better to me if an abstraction construct was introduced; but seeing you don't like the idea I guess I won't insist. Jan
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