Re: Wait free LW_SHARED acquisition - v0.2

On 2014-10-08 14:23:44 -0300, Alvaro Herrera wrote:
> Robert Haas wrote:
> > On Wed, Oct 8, 2014 at 8:47 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > > I don't see that as being relevant. The difference is an instruction or
> > > two - in the slow path we'll enter the kernel and sleep. This doesn't
> > > matter in comparison.
> > > And the code is *so* much more readable.
> >
> > I find the slist/dlist stuff actually quite difficult to get right
> > compared to a hand-rolled linked list. But the really big problem is
> > that the debugger can't do anything useful with it. You have to work
> > out the structure-member offset in order to walk the list and manually
> > cast to char *, adjust the pointer, and cast back. That sucks.
>
> As far as I recall you can get gdb to understand those pointer games
> by defining some structs or macros. Maybe we can improve by documenting
> this.

So, what makes it work for me (among other unrelated stuff) seems to be
the following in .gdbinit, defineing away some things that gdb doesn't
handle:
macro define __builtin_offsetof(T, F) ((int) &(((T *) 0)->F))
macro define __extension__
macro define AssertVariableIsOfTypeMacro(x, y) ((void)0)

Additionally I have "-ggdb -g3" in CFLAGS. That way gdb knows about
postgres' macros. At least if you're in the right scope.

As an example, the following works:
(gdb) p dlist_is_empty(&BackendList) ? NULL : dlist_head_element(Backend, elem, &BackendList)

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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