Multiprocessing Pool method hanging but Process method working

I'm trying to get the multiprocessing.Pool method to work. It initiates and shows all the cores are active, although ends up just stalled without completing.

To troubleshoot the problem, I first tried using a simple for loop and that worked (Method 1 below). I also tried using the multiprocessing.Process method, which also worked (Method 2 below).

Method 3 (below), however, just hangs. Where might I be going wrong in implementing the multiprocessing.Pool method?

import arcpy, os, glob, multiprocessing

# Method 1 works
inws = r'C:tempraster_data'
rasters = glob.glob(os.path.join(inws, "*.tif"))

def worker(raster):
    arcpy.arcpy.BuildRasterAttributeTable_management(raster)
    arcpy.AddField_management(raster, "Cover", "TEXT", "", "", 20)

for raster in rasters:
    worker(raster)

# Method 2 works
def mp_worker(raster):
    arcpy.arcpy.BuildRasterAttributeTable_management(raster)
    arcpy.AddField_management(raster, "Cover", "TEXT", "", "", 20)

if __name__ == '__main__':
    inws = r'C:tempraster_data'
    rasters = glob.glob(os.path.join(inws, "*.tif"))
    for raster in rasters:
        p = multiprocessing.Process(target=mp_worker(raster))
        p.start()
        p.join()

# Method 3 not working
def mp_worker(raster):
    arcpy.arcpy.BuildRasterAttributeTable_management(raster)
    arcpy.AddField_management(raster, "Cover", "TEXT", "", "", 20)

def mp_handler():
    p = multiprocessing.Pool(8)
    p.map(mp_worker, rasters)

if __name__ == '__main__':
    inws = r'C:tempraster_data'
    rasters = glob.glob(os.path.join(inws, "*.tif"))
    mp_handler()