Re: [PATCH][PING][PR gdb/19361] Fix invalid comparison functions

[Yuri Gribov <tetra2005@gmail.com>]

On Thu, Dec 31, 2015 at 12:35 AM, Pedro Alves <palves@redhat.com> wrote:
> On 12/30/2015 09:25 PM, Pedro Alves wrote:
>> On 12/30/2015 08:18 PM, Yuri Gribov wrote:
>>
>>> Sorry, I should have been more wordy about the actual problem. With
>>> current approach i.e.
>>>
>>>   if (pid1 == pgid1)
>>>     return -1;
>>>   else if (pid2 == pgid2)
>>>     return 1;
>>>
>>> comparison of two group leaders is not going to be symmetric:
>>>
>>>   cmp(lead_1, lead_2) == cmp(lead_2, lead_1) == -1
>>
>> Aaaaaaah, d'oh!  Thanks, it's obvious now, yes, we fail to consider
>> the case of both elements being leaders.  I couldn't see that
>> even after staring at the code for a while.  That hunk is OK as
>> is then.  (Please clarify this in the commit log.)
>
> Wait, no we don't...  If both are leaders when you get there, then
> they must have different pgid's, and that case is handled before:
>
>   /* Sort by PGID.  */
>   if (pgid1 < pgid2)
>     return -1;
>   else if (pgid1 > pgid2)
>     return 1;
>   else
>
> But let's assume not.  Let's assume we see two leaders when you get to the
> code in question.  That means they have pgid1==pgid2.  Since by definition
> being leader means pgid==pid, it must be that pid1 == pid2 as well.  That
> is, this is really about comparing equivalent elements.  Which
> brings us back again to:
>
>       /* Easier to check for equivalent element first.  */
>       if (pid1 == pid2)
>         return 0;
>
> Or am I confused again?

Hm, this sounds reasonable. Let me see if I can repro this bug and get
the exact inputs which caused the problem (unfortunately I didn't
collect them during testing).

-Y