Re: [PATCH] lib/igt_core: document the caveats of magic code blocks

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Mar 14, 2014 at 02:49:16PM +0000, Tvrtko Ursulin wrote:
> 
> On 03/14/2014 07:18 AM, Daniel Vetter wrote:
> [snip]
> >+ * # Magic Control Blocks
> >+ *
> >+ * i-g-t makes heavy use of C macros which serve as magic control blocks. They
> >+ * work fairly well and transparently but since C doesn't have full-blown
> >+ * closures there are caveats:
> >+ *
> >+ * - Asynchronous blocks which are used to spawn children internally use fork().
> >+ *   Which means that impossible control flow like jumping out of the control
> >+ *   block is possible, but it will horribly badly the i-g-t library code. And
> 
> I suggest to add a verb and reorder the "horribly badly" part of the
> sentence a bit. Or perhaps you tried to illustrate the program flow
> after jumping out of the control block? :)

I did frob the paragraph a bit before pushing. Can you please take a look
at what's committed and if you see some room for improvement send out
another patch?

> 
> >+ * - Code blocks with magic control flow are implemented with setjmp() and
> >+ *   longjmp(). This applies to #igt_fixture and #igt_subtest blocks and all the
> >+ *   three variants to finish test: igt_success(), igt_skip() and igt_fail().
> >+ *   Mostly this is of no concern, except when such a control block changes
> >+ *   stack variables defined in the same function as the control block resides.
> >+ *   Any store/load behaviour after a longjmp() is ill-defined for these
> >+ *   variables. Avoid such code.
> >+ *
> >+ *   Quoting the man page for longjmp():
> >+ *
> >+ *   "The values of automatic variables are unspecified after a call to
> >+ *   longjmp() if they meet all the following criteria:"
> >+ *    - "they are local to the function that made the corresponding setjmp() call;
> >+ *    - "their values are changed between the calls to setjmp() and longjmp(); and
> >+ *    - "they are not declared as volatile."
> >   */
> 
> So all variables which are getting initialised in igt_fixture should
> be moved out to file scope? Gcc does warn about potential clobbering
> although I haven't exactly gotten my head round understanding if and
> when it can really happen with igt_fixture.

Actually igt_fixture and igt_subtest, but usually it's igt_fixtures where
something gets set up and then clobbered. What I usually do is push them
out to global scope, but not at the top of the file but only right above
igt_main. That tends to avoid gcc warnings about shadowing.

Woud it be useful to add this bkm to the docs, too?

Thanks for the feedback, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch