Re: Profile mode maintenance patch

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 23 September 2014 22:19, FranÃois Dumont wrote:
> On 23/09/2014 13:27, Jonathan Wakely wrote:
>>
>> On 21/09/14 23:29 +0200, FranÃois Dumont wrote:
>>>
>>>    With all those modifications I have been able to run all testsuite in
>>> profile mode with success.
>>
>>
>> I've looked over the patch and it looks fine.
>>
>> I don't know the details of the Profile Mode, so if you're happy that
>> these changes are an improvement and all tests pass then that's good
>> enough for me.
>>
>>>    Ok to commit ?
>>
>>
>> Yes, OK for trunk - thanks very much.
>>
>>
>
> Ok but could you just let me know what you think of this method:
>
>   template<typename __object_info, typename __stack_info>
>     __object_info*
>     __trace_base<__object_info, __stack_info>::
>     __add_object(const __object_info& __info)
>     {
>       if (__max_mem() != 0 && __objects_byte_size >= __max_mem())
>     {
>       delete __info.__stack();
>       return 0;
>     }
>
>       __object_info* __ret = new(std::nothrow) __object_info(__info);
>       if (!__ret)
>     {
>       delete __info.__stack();
>       return 0;
>     }
>
>       __gnu_cxx::__atomic_add(&__objects_byte_size, sizeof(__object_info));
>       return __ret;
>     }
>
> This method can be called from several threads. I check condition accessing
> __object_byte_size and then update it with an atomic operation to make sure
> it stays consistent. Does it look ok to you too ?

It can result in a data race if the non-atomic access happens
concurrently with the atomic update.

The read should be an atomic load to solve that, but we don't have a
dispatch function in <ext/atomicity.h> for atomic loads.