Re: [PATCH 2/3] btrfs: add const function attribute

[David Sterba <dsterba@suse.cz>]

On Wed, Oct 02, 2019 at 02:07:50PM +0300, Nikolay Borisov wrote:
> > +struct list_head * __attribute_const__ btrfs_get_fs_uuids(void)
> 
> I'm not entirely sure this function is cons. According to the manual:
> 
> Calls to functions whose return value is not affected by changes to the
> observable state of the program and that have no observable effects on
> such state other than to return a value may lend themselves to
> optimizations such as common subexpression elimination.
> 
> The const attribute prohibits a function from reading objects that
> affect its return value between successive invocations. However,
> functions declared with the attribute can safely read objects that do
> not change their return value, such as non-volatile constants.
> 
> 
> My doubt stems from the fact this function actually references outside
> memory, namely gets the ptr to fs_uuids. There is a specific remark not
> to use const when the function takes a ptr argument but it doesn't say
> anything when getting a ptr from a global var.

The fs_uuids are a non-volatile constant. It does not change accross the
lifetime of the module, so all code executed will always see the same
value.