#!/usr/bin/env python3
# Copyright (c) Meta Platforms, Inc. and affiliates.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import re
from collections import defaultdict
from enum import Enum
import gdb
class DiGraph:
"""
Adapted from networkx: http://networkx.github.io/
Represents a directed graph. Edges can store (key, value) attributes.
"""
def __init__(self):
# Map of node -> set of nodes
self.adjacency_map = {}
# Map of (node1, node2) -> map string -> arbitrary attribute
# This will not be copied in subgraph()
self.attributes_map = {}
def neighbors(self, node):
return self.adjacency_map.get(node, set())
def edges(self):
edges = []
for node, neighbors in self.adjacency_map.items():
for neighbor in neighbors:
edges.append((node, neighbor))
return edges
def nodes(self):
return self.adjacency_map.keys()
def attributes(self, node1, node2):
return self.attributes_map[(node1, node2)]
def add_edge(self, node1, node2, **kwargs):
if node1 not in self.adjacency_map:
self.adjacency_map[node1] = set()
if node2 not in self.adjacency_map:
self.adjacency_map[node2] = set()
self.adjacency_map[node1].add(node2)
self.attributes_map[(node1, node2)] = kwargs
def remove_node(self, node):
self.adjacency_map.pop(node, None)
for _, neighbors in self.adjacency_map.items():
neighbors.discard(node)
def subgraph(self, nodes):
graph = DiGraph()
for node in nodes:
for neighbor in self.neighbors(node):
if neighbor in nodes:
graph.add_edge(node, neighbor)
return graph
def node_link_data(self):
"""
Returns the graph as a dictionary in a format that can be
serialized.
"""
data = {
"directed": True,
"multigraph": False,
"graph": {},
"links": [],
"nodes": [],
}
# Do one pass to build a map of node -> position in nodes
node_to_number = {}
for node in self.adjacency_map.keys():
node_to_number[node] = len(data["nodes"])
data["nodes"].append({"id": node})
# Do another pass to build the link information
for node, neighbors in self.adjacency_map.items():
for neighbor in neighbors:
link = self.attributes_map[(node, neighbor)].copy()
link["source"] = node_to_number[node]
link["target"] = node_to_number[neighbor]
data["links"].append(link)
return data
def strongly_connected_components(G): # noqa: C901
"""
Adapted from networkx: http://networkx.github.io/
Parameters
----------
G : DiGraph
Returns
-------
comp : generator of sets
A generator of sets of nodes, one for each strongly connected
component of G.
"""
preorder = {}
lowlink = {}
scc_found = {}
scc_queue = []
i = 0 # Preorder counter
for source in G.nodes():
if source not in scc_found:
queue = [source]
while queue:
v = queue[-1]
if v not in preorder:
i = i + 1
preorder[v] = i
done = 1
v_nbrs = G.neighbors(v)
for w in v_nbrs:
if w not in preorder:
queue.append(w)
done = 0
break
if done == 1:
lowlink[v] = preorder[v]
for w in v_nbrs:
if w not in scc_found:
if preorder[w] > preorder[v]:
lowlink[v] = min([lowlink[v], lowlink[w]])
else:
lowlink[v] = min([lowlink[v], preorder[w]])
queue.pop()
if lowlink[v] == preorder[v]:
scc_found[v] = True
scc = {v}
while scc_queue and preorder[scc_queue[-1]] > preorder[v]:
k = scc_queue.pop()
scc_found[k] = True
scc.add(k)
yield scc
else:
scc_queue.append(v)
def simple_cycles(G): # noqa: C901
"""
Adapted from networkx: http://networkx.github.io/
Parameters
----------
G : DiGraph
Returns
-------
cycle_generator: generator
A generator that produces elementary cycles of the graph.
Each cycle is represented by a list of nodes along the cycle.
"""
def _unblock(thisnode, blocked, B):
stack = {thisnode}
while stack:
node = stack.pop()
if node in blocked:
blocked.remove(node)
stack.update(B[node])
B[node].clear()
# Johnson's algorithm requires some ordering of the nodes.
# We assign the arbitrary ordering given by the strongly connected comps
# There is no need to track the ordering as each node removed as processed.
# save the actual graph so we can mutate it here
# We only take the edges because we do not want to
# copy edge and node attributes here.
subG = G.subgraph(G.nodes())
sccs = list(strongly_connected_components(subG))
while sccs:
scc = sccs.pop()
# order of scc determines ordering of nodes
startnode = scc.pop()
# Processing node runs 'circuit' routine from recursive version
path = [startnode]
blocked = set() # vertex: blocked from search?
closed = set() # nodes involved in a cycle
blocked.add(startnode)
B = defaultdict(set) # graph portions that yield no elementary circuit
stack = [(startnode, list(subG.neighbors(startnode)))]
while stack:
thisnode, nbrs = stack[-1]
if nbrs:
nextnode = nbrs.pop()
if nextnode == startnode:
yield path[:]
closed.update(path)
elif nextnode not in blocked:
path.append(nextnode)
stack.append((nextnode, list(subG.neighbors(nextnode))))
closed.discard(nextnode)
blocked.add(nextnode)
continue
# done with nextnode... look for more neighbors
if not nbrs: # no more nbrs
if thisnode in closed:
_unblock(thisnode, blocked, B)
else:
for nbr in subG.neighbors(thisnode):
if thisnode not in B[nbr]:
B[nbr].add(thisnode)
stack.pop()
path.pop()
# done processing this node
subG.remove_node(startnode)
H = subG.subgraph(scc) # make smaller to avoid work in SCC routine
sccs.extend(list(strongly_connected_components(H)))
def find_cycle(graph):
"""
Looks for a cycle in the graph. If found, returns the first cycle.
If nodes a1, a2, ..., an are in a cycle, then this returns:
[(a1,a2), (a2,a3), ... (an-1,an), (an, a1)]
Otherwise returns an empty list.
"""
cycles = list(simple_cycles(graph))
if cycles:
nodes = cycles[0]
nodes.append(nodes[0])
edges = []
prev = nodes[0]
for node in nodes[1:]:
edges.append((prev, node))
prev = node
return edges
else:
return []
def get_stacktrace(thread_id):
"""
Returns the stack trace for the thread id as a list of strings.
"""
gdb.execute("thread %d" % thread_id, from_tty=False, to_string=True)
output = gdb.execute("bt", from_tty=False, to_string=True)
stacktrace_lines = output.strip().split("\n")
return stacktrace_lines
def is_thread_blocked_with_frame(
thread_id, top_line, expected_top_lines, expected_frame
):
"""
Returns True if we found expected_top_line in top_line, and
we found the expected_frame in the thread's stack trace.
"""
if all(expected not in top_line for expected in expected_top_lines):
return False
stacktrace_lines = get_stacktrace(thread_id)
return any(expected_frame in line for line in stacktrace_lines)
class MutexType(Enum):
"""Types of mutexes that we can detect deadlocks."""
PTHREAD_MUTEX_T = "pthread_mutex_t"
PTHREAD_RWLOCK_T = "pthread_rwlock_t"
@staticmethod
def get_mutex_type(thread_id, top_line):
"""
Returns the probable mutex type, based on the first line
of the thread's stack. Returns None if not found.
"""
WAITLIST = [
"__lll_lock_wait",
"futex_abstimed_wait",
"futex_abstimed_wait_cancelable",
"futex_reltimed_wait",
"futex_reltimed_wait_cancelable",
"futex_wait",
"futex_wait_cancelable",
]
if is_thread_blocked_with_frame(thread_id, top_line, WAITLIST, "pthread_mutex"):
return MutexType.PTHREAD_MUTEX_T
if is_thread_blocked_with_frame(
thread_id, top_line, WAITLIST, "pthread_rwlock"
):
return MutexType.PTHREAD_RWLOCK_T
return None
@staticmethod
def get_mutex_owner_and_address_func_for_type(mutex_type):
"""
Returns a function to resolve the mutex owner and address for
the given type. The returned function f has the following
signature:
f: args: (map of thread lwp -> thread id), blocked thread lwp
returns: (lwp of thread owning mutex, mutex address)
or (None, None) if not found.
Returns None if there is no function for this mutex_type.
"""
if mutex_type == MutexType.PTHREAD_MUTEX_T:
return get_pthread_mutex_t_owner_and_address
if mutex_type == MutexType.PTHREAD_RWLOCK_T:
return get_pthread_rwlock_t_owner_and_address
return None
def print_cycle(graph, lwp_to_thread_id, cycle):
"""Prints the threads and mutexes involved in the deadlock."""
for m, n in cycle:
print(
"Thread %d (LWP %d) is waiting on %s (0x%016x) held by "
"Thread %d (LWP %d)"
% (
lwp_to_thread_id[m],
m,
graph.attributes(m, n)["mutex_type"].value,
graph.attributes(m, n)["mutex"],
lwp_to_thread_id[n],
n,
)
)
def get_thread_info():
"""
Returns a pair of:
- map of LWP -> thread ID
- map of blocked threads LWP -> potential mutex type
"""
# LWP -> thread ID
lwp_to_thread_id = {}
# LWP -> potential mutex type it is blocked on
blocked_threads = {}
output = gdb.execute("info threads", from_tty=False, to_string=True)
lines = output.strip().split("\n")[1:]
regex = re.compile(r"[\s\*]*(\d+).*Thread.*\(LWP (\d+)\).*")
for line in lines:
try:
thread_id = int(regex.match(line).group(1))
thread_lwp = int(regex.match(line).group(2))
lwp_to_thread_id[thread_lwp] = thread_id
mutex_type = MutexType.get_mutex_type(thread_id, line)
if mutex_type:
blocked_threads[thread_lwp] = mutex_type
except Exception:
continue
return (lwp_to_thread_id, blocked_threads)
def get_pthread_mutex_t_owner_and_address(lwp_to_thread_id, thread_lwp):
"""
Finds the thread holding the mutex that this thread is blocked on.
Returns a pair of (lwp of thread owning mutex, mutex address),
or (None, None) if not found.
"""
# Go up the stack to the pthread_mutex_lock frame
gdb.execute(
"thread %d" % lwp_to_thread_id[thread_lwp], from_tty=False, to_string=True
)
gdb.execute("frame 1", from_tty=False, to_string=True)
# Get the owner of the mutex by inspecting the internal
# fields of the mutex.
try:
mutex_info = gdb.parse_and_eval("mutex").dereference()
mutex_owner_lwp = int(mutex_info["__data"]["__owner"])
return (mutex_owner_lwp, int(mutex_info.address))
except gdb.error:
return (None, None)
def get_pthread_rwlock_t_owner_and_address(lwp_to_thread_id, thread_lwp):
"""
If the thread is waiting on a write-locked pthread_rwlock_t, this will
return the pair of:
(lwp of thread that is write-owning the mutex, mutex address)
or (None, None) if not found, or if the mutex is read-locked.
"""
# Go up the stack to the pthread_rwlock_{rd|wr}lock frame
gdb.execute(
"thread %d" % lwp_to_thread_id[thread_lwp], from_tty=False, to_string=True
)
gdb.execute("frame 2", from_tty=False, to_string=True)
# Get the owner of the mutex by inspecting the internal
# fields of the mutex.
try:
rwlock_info = gdb.parse_and_eval("rwlock").dereference()
rwlock_data = rwlock_info["__data"]
field_names = ["__cur_writer", "__writer"]
fields = rwlock_data.type.fields()
field = [f for f in fields if f.name in field_names][0]
rwlock_owner_lwp = int(rwlock_data[field])
# We can only track the owner if it is currently write-locked.
# If it is not write-locked or if it is currently read-locked,
# possibly by multiple threads, we cannot find the owner.
if rwlock_owner_lwp != 0:
return (rwlock_owner_lwp, int(rwlock_info.address))
else:
return (None, None)
except gdb.error:
return (None, None)
class Deadlock(gdb.Command):
"""Detects deadlocks"""
def __init__(self):
super(Deadlock, self).__init__("deadlock", gdb.COMMAND_NONE)
def invoke(self, arg, from_tty):
"""Prints the threads and mutexes in a deadlock, if it exists."""
lwp_to_thread_id, blocked_threads = get_thread_info()
# Nodes represent threads. Edge (A,B) exists if thread A
# is waiting on a mutex held by thread B.
graph = DiGraph()
# Go through all the blocked threads and see which threads
# they are blocked on, and build the thread wait graph.
for thread_lwp, mutex_type in blocked_threads.items():
get_owner_and_address_func = (
MutexType.get_mutex_owner_and_address_func_for_type(mutex_type)
)
if not get_owner_and_address_func:
continue
mutex_owner_lwp, mutex_address = get_owner_and_address_func(
lwp_to_thread_id, thread_lwp
)
if mutex_owner_lwp and mutex_address:
graph.add_edge(
thread_lwp,
mutex_owner_lwp,
mutex=mutex_address,
mutex_type=mutex_type,
)
# A deadlock exists if there is a cycle in the graph.
cycle = find_cycle(graph)
if cycle:
print("Found deadlock!")
print_cycle(graph, lwp_to_thread_id, cycle)
else:
print("No deadlock detected. " "Do you have debug symbols installed?")
def load():
# instantiate the Deadlock command
Deadlock()
print('Type "deadlock" to detect deadlocks.')
def info():
return "Detect deadlocks"
if __name__ == "__main__":
load()