linux-kernel - Naming of tag operations in the XArray

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Date:   Fri, 15 Dec 2017 04:34:17 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Matthew Wilcox <mawilcox@...rosoft.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Jens Axboe <axboe@...nel.dk>,
        Rehas Sachdeva <aquannie@...il.com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-nilfs@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Naming of tag operations in the XArray

On Thu, Dec 14, 2017 at 08:22:14PM -0800, Matthew Wilcox wrote:
> On Mon, Dec 11, 2017 at 03:10:22PM -0800, Randy Dunlap wrote:
> > > +A freshly-initialised XArray contains a ``NULL`` pointer at every index.
> > > +Each non-``NULL`` entry in the array has three bits associated with
> > > +it called tags.  Each tag may be flipped on or off independently of
> > > +the others.  You can search for entries with a given tag set.
> > 
> > Only tags that are set, or search for entries with some tag(s) cleared?
> > Or is that like a mathematical set?
> 
> hmm ...
> 
> "Each tag may be set or cleared independently of the others.  You can
> search for entries which have a particular tag set."
> 
> Doesn't completely remove the ambiguity, but I can't think of how to phrase
> that better ...

Thinking about this some more ...

At the moment, the pieces of the API which deal with tags look like this:

bool xa_tagged(const struct xarray *, xa_tag_t)
bool xa_get_tag(struct xarray *, unsigned long index, xa_tag_t);
void xa_set_tag(struct xarray *, unsigned long index, xa_tag_t);
void xa_clear_tag(struct xarray *, unsigned long index, xa_tag_t);
int xa_get_tagged(struct xarray *, void **dst, unsigned long start,
                        unsigned long max, unsigned int n, xa_tag_t);

bool xas_get_tag(const struct xa_state *, xa_tag_t);
void xas_set_tag(const struct xa_state *, xa_tag_t);
void xas_clear_tag(const struct xa_state *, xa_tag_t);
void *xas_find_tag(struct xa_state *, unsigned long max, xa_tag_t);
xas_for_each_tag(xas, entry, max, tag) { }

(at some point there will be an xa_for_each_tag too, there just hasn't
been a user yet).

I'm always ambivalent about using the word 'get' in an API because it has
two common meanings; (increment a refcount) and (return the state).  How
would people feel about these names instead:

bool xa_any_tagged(xa, tag);
bool xa_is_tagged(xa, index, tag);
void xa_tag(xa, index, tag);
void xa_untag(xa, index, tag);
int xa_select(xa, dst, start, max, n, tag);

bool xas_is_tagged(xas, tag);
void xas_tag(xas, tag);
void xas_untag(xas, tag);
void *xas_find_tag(xas, max, tag);
xas_for_each_tag(xas, entry, max, tag) { }

(the last two are unchanged)