diff --git a/waflib/Context.py b/waflib/Context.py
index 2fadde09..5148dd74 100644
--- a/waflib/Context.py
+++ b/waflib/Context.py
@@ -334,9 +334,8 @@ class Context(ctx):
 		cargs = {}
 		if 'timeout' in kw:
 			cargs['timeout'] = kw['timeout']
-			if kw['timeout'] is not None:
-				if kw['shell']:
-					Logs.warn('Shell commands cannot timeout %r', cmd)
+			if not 'start_new_session' in kw:
+				kw['start_new_session'] = True
 			del kw['timeout']
 		if 'input' in kw:
 			if kw['input']:
@@ -420,9 +419,8 @@ class Context(ctx):
 		cargs = {}
 		if 'timeout' in kw:
 			cargs['timeout'] = kw['timeout']
-			if kw['timeout'] is not None:
-				if kw['shell']:
-					Logs.warn('Shell commands cannot timeout %r', cmd)
+			if not 'start_new_session' in kw:
+				kw['start_new_session'] = True
 			del kw['timeout']
 		if 'input' in kw:
 			if kw['input']:
diff --git a/waflib/Utils.py b/waflib/Utils.py
index 9e84109d..bbab2c80 100644
--- a/waflib/Utils.py
+++ b/waflib/Utils.py
@@ -9,7 +9,7 @@ The portability fixes try to provide a consistent behavior of the Waf API
 through Python versions 2.5 to 3.X and across different platforms (win32, linux, etc)
 """
 
-import os, sys, errno, traceback, inspect, re, datetime, platform, base64
+import os, sys, errno, traceback, inspect, re, datetime, platform, base64, signal
 try:
 	import cPickle
 except ImportError:
@@ -865,7 +865,10 @@ def run_regular_process(cmd, kwargs, cargs={}):
 		try:
 			out, err = proc.communicate(**cargs)
 		except TimeoutExpired:
-			proc.kill()
+			try:
+				os.killpg(proc.pid, signal.SIGKILL)
+			except AttributeError:
+				proc.kill()
 			out, err = proc.communicate()
 			raise TimeoutExpired(proc.args, timeout=cargs['timeout'], output=out, stderr=err)
 		status = proc.returncode
@@ -874,7 +877,10 @@ def run_regular_process(cmd, kwargs, cargs={}):
 		try:
 			status = proc.wait(**cargs)
 		except TimeoutExpired as e:
-			proc.kill()
+			try:
+				os.killpg(proc.pid, signal.SIGKILL)
+			except AttributeError:
+				proc.kill()
 			proc.wait()
 			raise e
 	return status, out, err
diff --git a/waflib/processor.py b/waflib/processor.py
index b1ce9ac6..96e93cfa 100755
--- a/waflib/processor.py
+++ b/waflib/processor.py
@@ -2,7 +2,7 @@
 # encoding: utf-8
 # Thomas Nagy, 2016 (ita)
 
-import sys, traceback, base64
+import os, sys, traceback, base64, signal
 try:
 	import cPickle
 except ImportError:
@@ -34,7 +34,10 @@ def run():
 		try:
 			out, err = proc.communicate(**cargs)
 		except TimeoutExpired:
-			proc.kill()
+			try:
+				os.killpg(proc.pid, signal.SIGKILL)
+			except AttributeError:
+				proc.kill()
 			out, err = proc.communicate()
 			raise TimeoutExpired(proc.args, timeout=cargs['timeout'], output=out, stderr=err)
 		ret = proc.returncode