linux-kernel - Re: [PATCH 05/44] kdbus: Add comment on merging free pool slices

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Message-ID: <20151009181133.GG2189@localhost.localdomain>
Date:	Fri, 9 Oct 2015 21:11:33 +0300
From:	Sergei Zviagintsev <sergei@...v.net>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Daniel Mack <daniel@...que.org>,
	David Herrmann <dh.herrmann@...glemail.com>,
	Djalal Harouni <tixxdz@...ndz.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/44] kdbus: Add comment on merging free pool slices

Hi,

On Thu, Oct 08, 2015 at 03:50:46PM +0200, David Herrmann wrote:
> Hi
> 
> On Thu, Oct 8, 2015 at 1:31 PM, Sergei Zviagintsev <sergei@...v.net> wrote:
> > Add comment on why we remove the same slice from free slices tree and
> > then add it back again when merging the slice to be released with
> > previous free slice.
> >
> > Signed-off-by: Sergei Zviagintsev <sergei@...v.net>
> > ---
> >  ipc/kdbus/pool.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/ipc/kdbus/pool.c b/ipc/kdbus/pool.c
> > index 84afe96fbc22..c26ef963d8d1 100644
> > --- a/ipc/kdbus/pool.c
> > +++ b/ipc/kdbus/pool.c
> > @@ -304,6 +304,12 @@ static void __kdbus_pool_slice_release(struct kdbus_pool_slice *slice)
> >                 s = list_entry(slice->entry.prev,
> >                                struct kdbus_pool_slice, entry);
> >                 if (s->free) {
> > +                       /*
> > +                        * As size of slice increases after merge and free
> > +                        * slices tree is ordered by slice size, we have to
> > +                        * remove the slice from free slices tree and then add
> > +                        * it again to keep the tree balanced.
> > +                        */
> 
> Mhh, isn't this obvious? "slices_free" is ordered by s->size, so a
> change of the key requires a re-insert. If you disagree, maybe keep it
> simple:
> 
>         /* s->size changed, re-insert slice in rbtree */

It wasn't obvious for me from the first sight (but don't tell anyone),
so I decided that some words on why we're doing re-insert would be
helpful. It's up to you to decide whether it would be useful for the
others. BTW, I find your shorter variant better.

> 
> Thanks
> David
> 
> >                         rb_erase(&s->rb_node, &pool->slices_free);
> >                         list_del(&slice->entry);
> >                         s->size += slice->size;
> > --
> > 1.8.3.1
> >
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