Re: test_stack on powerpc (power7)

On Tue, Jan 28, 2014 at 03:22:35PM -0600, Will Schmidt wrote:
> Hi All, 
>   I've been looking at the test_stack test case failure as seen on
> ppc64 / power7 based systems.    I don't have a fix, but believe I
> understand where the problem is occurring.  
> 
> The simplest case I've been able to duplicate is with three threads.
> As I've added debug to the code, the problem gets harder to nail down
> precisely, but this is what seems to be happening.
> 
> In the failure scenario:
>   The list appears OK during run_one_test() before and after
> AO_stack_pop() is called.  The thread is holding two entries in the t[i]
> array, and the list still looks OK. The list is damaged after the
> AO_stack_push() call is made.
> 
> Within AO_stack_push(),
> [src/atomic_ops_stack.c:AO_stack_pop_explicit_aux_require()]
> The malfunction seems to be triggered while one of the threads is
> between the "first=AO_load(list);" and the
> "AO_compare_and_swap_release(list,first,next);".  Either one or both of
> the other threads will have removed and replaced multiple elements, such
> that the compare and swap of list,first,next will pass the check, but
> the list entries, particularly the next pointer at first, has changed. 
> 
> This is referenced in the comment at that location:
>   /* Thus its next link cannot have changed out from under us, and we   */
>   /* removed exactly one entry and preserved the rest of the list.      */
>   /* Note that it is quite possible that an additional entry was        */
>   /* inserted and removed while we were running; this is OK since the   */
>   /* part of the list following first must have remained unchanged, and */
>   /* first must again have been at the head of the list when the        */
>   /* compare_and_swap succeeded.                                        */
> 
> which seem to be untrue in this case.
> 
> 
> The powerpc AO_* functions seem to be OK.  We'd prefer the gcc atomic
> builtins be used (http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html),
> (thats what they are there for), but I don't think that change 
> would help in this case.
> 
> My recommendation is that the test be rewritten to handle the case where
> first->next has changed underneath the current thread.
>  A shorter term fix would probably be to disable the test_stack test for
> power7 and newer processors, until it can be fixed. 

So does this mean the stack test is wrong?  I was worried that the actual
AO_ functions were wrong on powerpc.

-- 
Len Sorensen