Re: [PATCH v13.2 01/14] vpci: use per-domain PCI lock to protect vpci structure

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.02.2024 13:47, Stewart Hildebrand wrote:
> On 2/19/24 07:10, Jan Beulich wrote:
>> On 19.02.2024 12:47, Stewart Hildebrand wrote:
>>> @@ -895,6 +891,15 @@ int vpci_msix_arch_print(const struct vpci_msix *msix)
>>>  {
>>>      unsigned int i;
>>>  
>>> +    /*
>>> +     * Assert that d->pdev_list doesn't change. 
>>> ASSERT_PDEV_LIST_IS_READ_LOCKED
>>> +     * is not suitable here because it may allow either pcidevs_lock() or
>>> +     * d->pci_lock to be held, but here we rely on d->pci_lock being held, 
>>> not
>>> +     * pcidevs_lock().
>>> +     */
>>> +    ASSERT(rw_is_locked(&msix->pdev->domain->pci_lock));
>>> +    ASSERT(spin_is_locked(&msix->pdev->vpci->lock));
>>
>> There's no "d" in sight here, so it's a little odd that "d" is being talked
>> about. But I guess people can infer what's meant without too much trouble.
> 
> I can s/d->pci_lock/msix->pdev->domain->pci_lock/ for the next rev.

Or simply drop the d-s? That would be better for readability's sake,
I think.

>>> @@ -313,17 +316,36 @@ void vpci_dump_msi(void)
>>>                  {
>>>                      /*
>>>                       * On error vpci_msix_arch_print will always return 
>>> without
>>> -                     * holding the lock.
>>> +                     * holding the locks.
>>>                       */
>>>                      printk("unable to print all MSI-X entries: %d\n", rc);
>>> -                    process_pending_softirqs();
>>> -                    continue;
>>> +                    goto pdev_done;
>>>                  }
>>>              }
>>>  
>>> +            /*
>>> +             * Unlock locks to process pending softirqs. This is
>>> +             * potentially unsafe, as d->pdev_list can be changed in
>>> +             * meantime.
>>> +             */
>>>              spin_unlock(&pdev->vpci->lock);
>>> +            read_unlock(&d->pci_lock);
>>> +        pdev_done:
>>>              process_pending_softirqs();
>>> +            if ( !read_trylock(&d->pci_lock) )
>>> +            {
>>> +                printk("unable to access other devices for the domain\n");
>>> +                goto domain_done;
>>> +            }
>>>          }
>>> +        read_unlock(&d->pci_lock);
>>> +    domain_done:
>>> +        /*
>>> +         * We need this label at the end of the loop, but some
>>> +         * compilers might not be happy about label at the end of the
>>> +         * compound statement so we adding an empty statement here.
>>> +         */
>>> +        ;
>>
>> As to "some compilers": Are there any which accept a label not followed
>> by a statement? Depending on the answer, this comment may be viewed as
>> superfluous. Or else I'd ask about wording: Besides a grammar issue I
>> also don't view it as appropriate that a comment talks about "adding"
>> something when its adjacent code that is meant. That something is there
>> when the comment is there, hence respective wording should imo be used.
> 
> It seems like hit or miss whether gcc would accept it or not (prior
> discussion at [1]). I agree the comment is rather lengthy for what it's
> trying to convey. I'd be happy to either remove the comment or reduce
> it to:
> 
>     domain_done:
>         ; /* Empty statement to make some compilers happy */
> 
> [1] 
> https://lore.kernel.org/xen-devel/98b8c131-b0b9-f46c-5f46-c2136f2e3b4e@xxxxxxx/

This earlier discussion only proves that there is at least one compiler
objecting. There's no proof there that any compiler exists which, as a
language extension, actually permits such syntax. Yet if the comment
was purely about normal language syntax, then imo it should be zapped
altogether, not just be shrunk.

Jan