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src: fix sporadic deadlock in SIGUSR1 handler · nodejs/node@db2b23f · GitHub

/ node Public

Commit db2b23f

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bnoordhuis
authored and
Myles Borins
committed
src: fix sporadic deadlock in SIGUSR1 handler
Calling v8::Debug::DebugBreak() directly from the signal handler is unsafe because said function tries to grab a mutex. Work around that by starting a watchdog thread that is woken up from the signal handler, which then calls v8::Debug::DebugBreak(). Using a watchdog thread also removes the need to use atomic operations so this commit does away with that. Fixes: #5368 PR-URL: #5904 Reviewed-By: Anna Henningsen <anna@addaleax.net>
1 parent 8557597 commit db2b23f

2 files changed

Lines changed: 74 additions & 59 deletions

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‎src/atomic-polyfill.h‎

Lines changed: 0 additions & 18 deletions
This file was deleted.

‎src/node.cc‎

Lines changed: 74 additions & 41 deletions
Original file line numberDiff line numberDiff line change
@@ -73,6 +73,7 @@
7373
#define umask _umask
7474
typedef int mode_t;
7575
#else
76+
#include <pthread.h>
7677
#include <sys/resource.h> // getrlimit, setrlimit
7778
#include <unistd.h> // setuid, getuid
7879
#endif
@@ -89,14 +90,6 @@ typedef int mode_t;
8990
extern char **environ;
9091
#endif
9192

92-
#ifdef __APPLE__
93-
#include "atomic-polyfill.h" // NOLINT(build/include_order)
94-
namespace node { template <typename T> using atomic = nonstd::atomic<T>; }
95-
#else
96-
#include <atomic>
97-
namespace node { template <typename T> using atomic = std::atomic<T>; }
98-
#endif
99-
10093
namespace node {
10194

10295
using v8::Array;
@@ -166,9 +159,13 @@ static double prog_start_time;
166159
static bool debugger_running;
167160
static uv_async_t dispatch_debug_messages_async;
168161

169-
static node::atomic<Isolate*> node_isolate;
162+
static uv_mutex_t node_isolate_mutex;
163+
static v8::Isolate* node_isolate;
170164
static v8::Platform* default_platform;
171165

166+
#ifdef __POSIX__
167+
static uv_sem_t debug_semaphore;
168+
#endif
172169

173170
static void PrintErrorString(const char* format, ...) {
174171
va_list ap;
@@ -3538,44 +3535,40 @@ static void EnableDebug(Environment* env) {
35383535

35393536
// Called from an arbitrary thread.
35403537
static void TryStartDebugger() {
3541-
// Call only async signal-safe functions here! Don't retry the exchange,
3542-
// it will deadlock when the thread is interrupted inside a critical section.
3543-
if (auto isolate = node_isolate.exchange(nullptr)) {
3538+
uv_mutex_lock(&node_isolate_mutex);
3539+
if (auto isolate = node_isolate) {
35443540
v8::Debug::DebugBreak(isolate);
35453541
uv_async_send(&dispatch_debug_messages_async);
3546-
CHECK_EQ(nullptr, node_isolate.exchange(isolate));
35473542
}
3543+
uv_mutex_unlock(&node_isolate_mutex);
35483544
}
35493545

35503546

35513547
// Called from the main thread.
35523548
static void DispatchDebugMessagesAsyncCallback(uv_async_t* handle) {
3553-
// Synchronize with signal handler, see TryStartDebugger.
3554-
Isolate* isolate;
3555-
do {
3556-
isolate = node_isolate.exchange(nullptr);
3557-
} while (isolate == nullptr);
3549+
uv_mutex_lock(&node_isolate_mutex);
3550+
if (auto isolate = node_isolate) {
3551+
if (debugger_running == false) {
3552+
fprintf(stderr, "Starting debugger agent.\n");
35583553

3559-
if (debugger_running == false) {
3560-
fprintf(stderr, "Starting debugger agent.\n");
3554+
HandleScope scope(isolate);
3555+
Environment* env = Environment::GetCurrent(isolate);
3556+
Context::Scope context_scope(env->context());
35613557

3562-
HandleScope scope(isolate);
3563-
Environment* env = Environment::GetCurrent(isolate);
3564-
Context::Scope context_scope(env->context());
3558+
StartDebug(env, false);
3559+
EnableDebug(env);
3560+
}
35653561

3566-
StartDebug(env, false);
3567-
EnableDebug(env);
3562+
Isolate::Scope isolate_scope(isolate);
3563+
v8::Debug::ProcessDebugMessages();
35683564
}
3569-
3570-
Isolate::Scope isolate_scope(isolate);
3571-
v8::Debug::ProcessDebugMessages();
3572-
CHECK_EQ(nullptr, node_isolate.exchange(isolate));
3565+
uv_mutex_unlock(&node_isolate_mutex);
35733566
}
35743567

35753568

35763569
#ifdef __POSIX__
35773570
static void EnableDebugSignalHandler(int signo) {
3578-
TryStartDebugger();
3571+
uv_sem_post(&debug_semaphore);
35793572
}
35803573

35813574

@@ -3614,11 +3607,46 @@ void DebugProcess(const FunctionCallbackInfo<Value>& args) {
36143607
}
36153608

36163609

3610+
inline void* DebugSignalThreadMain(void* unused) {
3611+
for (;;) {
3612+
uv_sem_wait(&debug_semaphore);
3613+
TryStartDebugger();
3614+
}
3615+
return nullptr;
3616+
}
3617+
3618+
36173619
static int RegisterDebugSignalHandler() {
3618-
// FIXME(bnoordhuis) Should be per-isolate or per-context, not global.
3620+
// Start a watchdog thread for calling v8::Debug::DebugBreak() because
3621+
// it's not safe to call directly from the signal handler, it can
3622+
// deadlock with the thread it interrupts.
3623+
CHECK_EQ(0, uv_sem_init(&debug_semaphore, 0));
3624+
pthread_attr_t attr;
3625+
CHECK_EQ(0, pthread_attr_init(&attr));
3626+
// Don't shrink the thread's stack on FreeBSD. Said platform decided to
3627+
// follow the pthreads specification to the letter rather than in spirit:
3628+
// https://lists.freebsd.org/pipermail/freebsd-current/2014-March/048885.html
3629+
#ifndef __FreeBSD__
3630+
CHECK_EQ(0, pthread_attr_setstacksize(&attr, PTHREAD_STACK_MIN));
3631+
#endif // __FreeBSD__
3632+
CHECK_EQ(0, pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED));
3633+
sigset_t sigmask;
3634+
sigfillset(&sigmask);
3635+
CHECK_EQ(0, pthread_sigmask(SIG_SETMASK, &sigmask, &sigmask));
3636+
pthread_t thread;
3637+
const int err =
3638+
pthread_create(&thread, &attr, DebugSignalThreadMain, nullptr);
3639+
CHECK_EQ(0, pthread_sigmask(SIG_SETMASK, &sigmask, nullptr));
3640+
CHECK_EQ(0, pthread_attr_destroy(&attr));
3641+
if (err != 0) {
3642+
fprintf(stderr, "node[%d]: pthread_create: %s\n", getpid(), strerror(err));
3643+
fflush(stderr);
3644+
// Leave SIGUSR1 blocked. We don't install a signal handler,
3645+
// receiving the signal would terminate the process.
3646+
return -err;
3647+
}
36193648
RegisterSignalHandler(SIGUSR1, EnableDebugSignalHandler);
36203649
// Unblock SIGUSR1. A pending SIGUSR1 signal will now be delivered.
3621-
sigset_t sigmask;
36223650
sigemptyset(&sigmask);
36233651
sigaddset(&sigmask, SIGUSR1);
36243652
CHECK_EQ(0, pthread_sigmask(SIG_UNBLOCK, &sigmask, nullptr));
@@ -3860,6 +3888,8 @@ void Init(int* argc,
38603888
// Make inherited handles noninheritable.
38613889
uv_disable_stdio_inheritance();
38623890

3891+
CHECK_EQ(0, uv_mutex_init(&node_isolate_mutex));
3892+
38633893
// init async debug messages dispatching
38643894
// Main thread uses uv_default_loop
38653895
CHECK_EQ(0, uv_async_init(uv_default_loop(),
@@ -4146,15 +4176,18 @@ static void StartNodeInstance(void* arg) {
41464176
params.code_event_handler = vTune::GetVtuneCodeEventHandler();
41474177
#endif
41484178
Isolate* isolate = Isolate::New(params);
4179+
4180+
uv_mutex_lock(&node_isolate_mutex);
4181+
if (instance_data->is_main()) {
4182+
CHECK_EQ(node_isolate, nullptr);
4183+
node_isolate = isolate;
4184+
}
4185+
uv_mutex_unlock(&node_isolate_mutex);
4186+
41494187
if (track_heap_objects) {
41504188
isolate->GetHeapProfiler()->StartTrackingHeapObjects(true);
41514189
}
41524190

4153-
// Fetch a reference to the main isolate, so we have a reference to it
4154-
// even when we need it to access it from another (debugger) thread.
4155-
if (instance_data->is_main())
4156-
CHECK_EQ(nullptr, node_isolate.exchange(isolate));
4157-
41584191
{
41594192
Locker locker(isolate);
41604193
Isolate::Scope isolate_scope(isolate);
@@ -4218,10 +4251,10 @@ static void StartNodeInstance(void* arg) {
42184251
env = nullptr;
42194252
}
42204253

4221-
if (instance_data->is_main()) {
4222-
// Synchronize with signal handler, see TryStartDebugger.
4223-
while (isolate != node_isolate.exchange(nullptr)); // NOLINT
4224-
}
4254+
uv_mutex_lock(&node_isolate_mutex);
4255+
if (node_isolate == isolate)
4256+
node_isolate = nullptr;
4257+
uv_mutex_unlock(&node_isolate_mutex);
42254258

42264259
CHECK_NE(isolate, nullptr);
42274260
isolate->Dispose();

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