Re: [PATCH v6 03/13] vpci: move lock outside of struct vpci

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.02.2022 08:35, Oleksandr Andrushchenko wrote:
> 1.1. Semi read lock upgrade in modify bars
> --------------------------------------------------------------
> In this case both vpci_read and vpci_write take a read lock and when it comes
> to modify_bars:
> 
> 1. read_unlock(d->vpci_lock)
> 2. write_lock(d->vpci_lock)
> 3. Check that pdev->vpci is still available and is the same object:
> if (pdev->vpci && (pdev->vpci == old_vpci) )
> {
>      /* vpci structure is valid and can be used. */
> }
> else
> {
>      /* vpci has gone, return an error. */
> }
> 
> Pros:
> - no per-device vpci lock is needed?
> - solves overlap code ABBA in modify_bars
> - readers and writers are NOT serialized
> - NO need to carefully select read paths, so they are guaranteed not to lead
>    to lock upgrade use-cases
> 
> Cons:
> - ???

The "pdev->vpci == old_vpci" is fragile: The struct may have got re-
allocated, and it just so happened that the two pointers are identical.

Same then for the subsequent variant 2.

Jan