Re: [PATCH v11 02/17] pci: introduce per-domain PCI rwlock

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.12.2023 02:27, Volodymyr Babchuk wrote:
> Add per-domain d->pci_lock that protects access to
> d->pdev_list. Purpose of this lock is to give guarantees to VPCI code
> that underlying pdev will not disappear under feet. This is a rw-lock,
> but this patch adds only write_lock()s. There will be read_lock()
> users in the next patches.
> 
> This lock should be taken in write mode every time d->pdev_list is
> altered. All write accesses also should be protected by pcidevs_lock()
> as well. Idea is that any user that wants read access to the list or
> to the devices stored in the list should use either this new
> d->pci_lock or old pcidevs_lock(). Usage of any of this two locks will
> ensure only that pdev of interest will not disappear from under feet
> and that the pdev still will be assigned to the same domain. Of
> course, any new users should use pcidevs_lock() when it is
> appropriate (e.g. when accessing any other state that is protected by
> the said lock). In case both the newly introduced per-domain rwlock
> and the pcidevs lock is taken, the latter must be acquired first.

Shouldn't an abridged version of this also be put ...

> --- a/xen/include/xen/sched.h
> +++ b/xen/include/xen/sched.h
> @@ -461,6 +461,7 @@ struct domain
>  
>  #ifdef CONFIG_HAS_PCI
>      struct list_head pdev_list;
> +    rwlock_t pci_lock;
>  #endif

... here, for easy future reference? As locking is going to become
more complicated, documenting lock nesting rules in code is imo pretty
important.

Jan