[Mesa-dev] [PATCH] glsl/list: make nodes safe for double removal, etc.

[Iago Toral]

On Mon, 2016-04-04 at 11:04 -0400, Rob Clark wrote:
> On Mon, Apr 4, 2016 at 10:34 AM, Iago Toral <itoral at igalia.com> wrote:
> > On Sat, 2016-04-02 at 17:09 -0400, Rob Clark wrote:
> >> From: Rob Clark <robclark at freedesktop.org>
> >>
> >> It's no extra overhead to do a _self_link() and it eliminates a class of
> >> potential problems.
> >
> > it can also hide actual programming mistakes that would otherwise be
> > immediately visible... does this actually help something specific?
> 
> Well, basically it avoids needing to explicitly do a _self_link()
> after removing a node in cases where you know (for example) that you
> might end up removing multiple times.  The kernel list implementation
> does have separate list_del() and list_del_init(), which would be a
> different possible way to go.
> 
> But in my experience the programming mistakes that this would hide are
> simply cases where you wanted to do list_del_init() instead of
> list_del(), so I'm curious about which other cases you are worried
> about.

I was thinking about incorrect double deletions of the same node. If we
are never supposed to do that this would be hiding a programming
mistake. In any case, I don't have a strong preference myself, I was
just curious about the reasons for the change.

> Anyways, this patch doesn't solve something in particular, it is
> mostly just a response to a comment Jason made about my usage of
> immediate _self_link() after removal on another patch.

I think having two functions for this, one that sets the pointers to
NULL and one that self links is a better solution since you can choose
the one that makes sense for what you are actually trying to do, but I
don't have a strong preference, if others like this approach I am fine
with it too.

Iago

> >
> >> Signed-off-by: Rob Clark <robclark at freedesktop.org>
> >> Dared-by: Jason Ekstrand <jason at jlekstrand.net>
> >> ---
> >>  src/compiler/glsl/list.h | 15 +++++++--------
> >>  1 file changed, 7 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/src/compiler/glsl/list.h b/src/compiler/glsl/list.h
> >> index a1c4d82..77e1f67 100644
> >> --- a/src/compiler/glsl/list.h
> >> +++ b/src/compiler/glsl/list.h
> >> @@ -165,19 +165,18 @@ exec_node_get_prev(struct exec_node *n)
> >>  }
> >>
> >>  static inline void
> >> -exec_node_remove(struct exec_node *n)
> >> +exec_node_self_link(struct exec_node *n)
> >>  {
> >> -   n->next->prev = n->prev;
> >> -   n->prev->next = n->next;
> >> -   n->next = NULL;
> >> -   n->prev = NULL;
> >> +   n->next = n;
> >> +   n->prev = n;
> >>  }
> >>
> >>  static inline void
> >> -exec_node_self_link(struct exec_node *n)
> >> +exec_node_remove(struct exec_node *n)
> >>  {
> >> -   n->next = n;
> >> -   n->prev = n;
> >> +   n->next->prev = n->prev;
> >> +   n->prev->next = n->next;
> >> +   exec_node_self_link(n);
> >>  }
> >>
> >>  static inline void
> >
> >
>