malloc: Trim unused arenas on thread exit

[Florian Weimer]

On 11/15/2017 03:17 PM, Siddhesh Poyarekar wrote:
> On Wednesday 15 November 2017 07:40 PM, Florian Weimer wrote:
>>
>> I assumed the comment was sufficient:
>>
>>>>  Â  static void __attribute__ ((section ("__libc_thread_freeres_fn")))
>>>>  Â  arena_thread_freeres (void)
>>>>  Â  {
>>>> +  /* Shut down the thread cache first.  This could deallocate data for
>>>> +Â Â Â Â  the thread arena, so do this before we put the arena on the free
>>>> +     list.  */
>>>> +Â  tcache_thread_shutdown ();
>>
>> If it is not, we need to put more documentation in the code itself.
> 
> The comment describes the current behaviour while the commit log should
> describe the change, something like:
> 
> "Call tcache destructor in arena_thread_freeres
> 
> Having separate cleanup functions for arena and tcache could result in
> the tcache freeres function being called later and thus not deallocate
> data for the thread arena.  Avoid this by calling the tcache cleanup
> function from within arena_thread_freeres."

But I have not verified that this actually happens, and we agreed that 
it was a good change nevertheless.

Thanks,
Florian