read_hdf crash python process when use it in multithread code
one of my folder contains multiple h5 files, and I tried to load them into dataframes and then concat these df into one.

the python process crashes when the num_tasks>1, if I debug thread by thread, it works, in another, it crashes simply when two threads run at the same time, even though they read different files.

```
from multiprocessing.pool import ThreadPool
import pandas as pd 

num_tasks=2
def readjob(x):
    path = x
    return pd.read_hdf(path,"df",mode='r')

pool = ThreadPool(num_tasks)
results = pool.map(readjob,files)
```
