[Intel-gfx] [PATCH] lib/igt_core: document the caveats of magic code blocks

[Tvrtko Ursulin]

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? :)

> + * - 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.

Tvrtko