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src: fix sporadic deadlock in SIGUSR1 handler
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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>
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@bnoordhuis
bnoordhuis authored and Myles Borins committed Jul 12, 2016
1 parent 07f1bb1 commit f886b01
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Showing 2 changed files with 74 additions and 59 deletions.
18 changes: 0 additions & 18 deletions src/atomic-polyfill.h
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This file was deleted.

115 changes: 74 additions & 41 deletions src/node.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -73,6 +73,7 @@
#define umask _umask
typedef int mode_t;
#else
#include <pthread.h>
#include <sys/resource.h> // getrlimit, setrlimit
#include <unistd.h> // setuid, getuid
#endif
Expand All @@ -89,14 +90,6 @@ typedef int mode_t;
extern char **environ;
#endif

#ifdef __APPLE__
#include "atomic-polyfill.h" // NOLINT(build/include_order)
namespace node { template <typename T> using atomic = nonstd::atomic<T>; }
#else
#include <atomic>
namespace node { template <typename T> using atomic = std::atomic<T>; }
#endif

namespace node {

using v8::Array;
Expand Down Expand Up @@ -166,9 +159,13 @@ static double prog_start_time;
static bool debugger_running;
static uv_async_t dispatch_debug_messages_async;

static node::atomic<Isolate*> node_isolate;
static uv_mutex_t node_isolate_mutex;
static v8::Isolate* node_isolate;
static v8::Platform* default_platform;

#ifdef __POSIX__
static uv_sem_t debug_semaphore;
#endif

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

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


// Called from the main thread.
static void DispatchDebugMessagesAsyncCallback(uv_async_t* handle) {
// Synchronize with signal handler, see TryStartDebugger.
Isolate* isolate;
do {
isolate = node_isolate.exchange(nullptr);
} while (isolate == nullptr);
uv_mutex_lock(&node_isolate_mutex);
if (auto isolate = node_isolate) {
if (debugger_running == false) {
fprintf(stderr, "Starting debugger agent.\n");

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

HandleScope scope(isolate);
Environment* env = Environment::GetCurrent(isolate);
Context::Scope context_scope(env->context());
StartDebug(env, false);
EnableDebug(env);
}

StartDebug(env, false);
EnableDebug(env);
Isolate::Scope isolate_scope(isolate);
v8::Debug::ProcessDebugMessages();
}

Isolate::Scope isolate_scope(isolate);
v8::Debug::ProcessDebugMessages();
CHECK_EQ(nullptr, node_isolate.exchange(isolate));
uv_mutex_unlock(&node_isolate_mutex);
}


#ifdef __POSIX__
static void EnableDebugSignalHandler(int signo) {
TryStartDebugger();
uv_sem_post(&debug_semaphore);
}


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


inline void* DebugSignalThreadMain(void* unused) {
for (;;) {
uv_sem_wait(&debug_semaphore);
TryStartDebugger();
}
return nullptr;
}


static int RegisterDebugSignalHandler() {
// FIXME(bnoordhuis) Should be per-isolate or per-context, not global.
// Start a watchdog thread for calling v8::Debug::DebugBreak() because
// it's not safe to call directly from the signal handler, it can
// deadlock with the thread it interrupts.
CHECK_EQ(0, uv_sem_init(&debug_semaphore, 0));
pthread_attr_t attr;
CHECK_EQ(0, pthread_attr_init(&attr));
// Don't shrink the thread's stack on FreeBSD. Said platform decided to
// follow the pthreads specification to the letter rather than in spirit:
// https://lists.freebsd.org/pipermail/freebsd-current/2014-March/048885.html
#ifndef __FreeBSD__
CHECK_EQ(0, pthread_attr_setstacksize(&attr, PTHREAD_STACK_MIN));
#endif // __FreeBSD__
CHECK_EQ(0, pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED));
sigset_t sigmask;
sigfillset(&sigmask);
CHECK_EQ(0, pthread_sigmask(SIG_SETMASK, &sigmask, &sigmask));
pthread_t thread;
const int err =
pthread_create(&thread, &attr, DebugSignalThreadMain, nullptr);
CHECK_EQ(0, pthread_sigmask(SIG_SETMASK, &sigmask, nullptr));
CHECK_EQ(0, pthread_attr_destroy(&attr));
if (err != 0) {
fprintf(stderr, "node[%d]: pthread_create: %s\n", getpid(), strerror(err));
fflush(stderr);
// Leave SIGUSR1 blocked. We don't install a signal handler,
// receiving the signal would terminate the process.
return -err;
}
RegisterSignalHandler(SIGUSR1, EnableDebugSignalHandler);
// Unblock SIGUSR1. A pending SIGUSR1 signal will now be delivered.
sigset_t sigmask;
sigemptyset(&sigmask);
sigaddset(&sigmask, SIGUSR1);
CHECK_EQ(0, pthread_sigmask(SIG_UNBLOCK, &sigmask, nullptr));
Expand Down Expand Up @@ -3860,6 +3888,8 @@ void Init(int* argc,
// Make inherited handles noninheritable.
uv_disable_stdio_inheritance();

CHECK_EQ(0, uv_mutex_init(&node_isolate_mutex));

// init async debug messages dispatching
// Main thread uses uv_default_loop
CHECK_EQ(0, uv_async_init(uv_default_loop(),
Expand Down Expand Up @@ -4146,15 +4176,18 @@ static void StartNodeInstance(void* arg) {
params.code_event_handler = vTune::GetVtuneCodeEventHandler();
#endif
Isolate* isolate = Isolate::New(params);

uv_mutex_lock(&node_isolate_mutex);
if (instance_data->is_main()) {
CHECK_EQ(node_isolate, nullptr);
node_isolate = isolate;
}
uv_mutex_unlock(&node_isolate_mutex);

if (track_heap_objects) {
isolate->GetHeapProfiler()->StartTrackingHeapObjects(true);
}

// Fetch a reference to the main isolate, so we have a reference to it
// even when we need it to access it from another (debugger) thread.
if (instance_data->is_main())
CHECK_EQ(nullptr, node_isolate.exchange(isolate));

{
Locker locker(isolate);
Isolate::Scope isolate_scope(isolate);
Expand Down Expand Up @@ -4218,10 +4251,10 @@ static void StartNodeInstance(void* arg) {
env = nullptr;
}

if (instance_data->is_main()) {
// Synchronize with signal handler, see TryStartDebugger.
while (isolate != node_isolate.exchange(nullptr)); // NOLINT
}
uv_mutex_lock(&node_isolate_mutex);
if (node_isolate == isolate)
node_isolate = nullptr;
uv_mutex_unlock(&node_isolate_mutex);

CHECK_NE(isolate, nullptr);
isolate->Dispose();
Expand Down

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