[PATCH v2][makedumpfile] Fix a data race in multi-threading mode (--num-threads=N)

[HAGIO KAZUHITO(萩尾 一仁)]

Hi Tao, Sourabh,

>> thank you for testing!  sorry one more thing,
>> does --num-threads=1 break the vmcore?
> 
> Yes:

Thank you for testing and information, certainly the race occurs between 
the main and sub-thread, I will check the code again.  If you could 
determine how it breaks the vmcore, please let me know.  It will be 
better to add the scenario to the commit log.

Thanks,
Kazu

> 
> [root at ibm-p10-01-lp45 makedumpfile]# ./makedumpfile -d 31 -l ~/vmcore /tmp/out7
> Copying data                                      : [100.0 %] /
>     eta: 0s
> 
> The dumpfile is saved to /tmp/out7.
> 
> makedumpfile Completed.
> [root at ibm-p10-01-lp45 makedumpfile]# ./makedumpfile --num-threads=1 -d
> 31 -l ~/vmcore /tmp/out8
> Copying data                                      : [100.0 %] -
>     eta: 0s
> Copying data                                      : [100.0 %] /
>     eta: 0s
> 
> The dumpfile is saved to /tmp/out8.
> 
> makedumpfile Completed.
> [root at ibm-p10-01-lp45 makedumpfile]# cmp /tmp/out7 /tmp/out8
> /tmp/out7 /tmp/out8 differ: byte 11119019, line 49418
> 
>>
>> Thanks,
>> Kazu
>