Re: [PATCH v2 12/17] xen/hypfs: add new enter() and exit() per node callbacks

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.12.2020 09:21, Juergen Gross wrote:
> In order to better support resource allocation and locking for dynamic
> hypfs nodes add enter() and exit() callbacks to struct hypfs_funcs.
> 
> The enter() callback is called when entering a node during hypfs user
> actions (traversing, reading or writing it), while the exit() callback
> is called when leaving a node (accessing another node at the same or a
> higher directory level, or when returning to the user).
> 
> For avoiding recursion this requires a parent pointer in each node.
> Let the enter() callback return the entry address which is stored as
> the last accessed node in order to be able to use a template entry for
> that purpose in case of dynamic entries.

I guess I'll learn in subsequent patches why this is necessary /
useful. Right now it looks odd for the function to simple return
the incoming argument, as this way it's clear the caller knows
the correct value already.

> @@ -100,11 +112,58 @@ static void hypfs_unlock(void)
>      }
>  }
>  
> +const struct hypfs_entry *hypfs_node_enter(const struct hypfs_entry *entry)
> +{
> +    return entry;
> +}
> +
> +void hypfs_node_exit(const struct hypfs_entry *entry)
> +{
> +}
> +
> +static int node_enter(const struct hypfs_entry *entry)
> +{
> +    const struct hypfs_entry **last = &this_cpu(hypfs_last_node_entered);
> +
> +    entry = entry->funcs->enter(entry);
> +    if ( IS_ERR(entry) )
> +        return PTR_ERR(entry);
> +
> +    ASSERT(!*last || *last == entry->parent);
> +
> +    *last = entry;
> +
> +    return 0;
> +}
> +
> +static void node_exit(const struct hypfs_entry *entry)
> +{
> +    const struct hypfs_entry **last = &this_cpu(hypfs_last_node_entered);
> +
> +    if ( !*last )
> +        return;

Under what conditions is this legitimate to happen? IOW shouldn't
there be an ASSERT_UNREACHABLE() here?

Jan