src/runtime/cgo/gcc_libinit_unix.c

// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

//go:build unix

// When cross-compiling with clang to linux/armv5, atomics are emulated
// and cause a compiler warning. This results in a build failure since
// cgo uses -Werror. See #65290.
#pragma GCC diagnostic ignored "-Wpragmas"
#pragma GCC diagnostic ignored "-Wunknown-warning-option"
#pragma GCC diagnostic ignored "-Watomic-alignment"

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include "libcgo.h"
#include "libcgo_unix.h"

static pthread_cond_t runtime_init_cond = PTHREAD_COND_INITIALIZER;
static pthread_mutex_t runtime_init_mu = PTHREAD_MUTEX_INITIALIZER;
static int runtime_init_done;

// pthread_g is a pthread specific key, for storing the g that binded to the C thread.
// The registered pthread_key_destructor will dropm, when the pthread-specified value g is not NULL,
// while a C thread is exiting.
static pthread_key_t pthread_g;
static void pthread_key_destructor(void* g);
uintptr_t x_cgo_pthread_key_created;
void (*x_crosscall2_ptr)(void (*fn)(void *), void *, int, size_t);

// The traceback function, used when tracing C calls.
static void (*cgo_traceback_function)(struct cgoTracebackArg*);

// The context function, used when tracing back C calls into Go.
static void (*cgo_context_function)(struct cgoContextArg*);

// The symbolizer function, used when symbolizing C frames.
static void (*cgo_symbolizer_function)(struct cgoSymbolizerArg*);

uintptr_t
_cgo_wait_runtime_init_done(void) {
	void (*pfn)(struct cgoContextArg*);
	int done;

	pfn = __atomic_load_n(&cgo_context_function, __ATOMIC_CONSUME);

	done = 2;
	if (__atomic_load_n(&runtime_init_done, __ATOMIC_CONSUME) != done) {
		pthread_mutex_lock(&runtime_init_mu);
		while (__atomic_load_n(&runtime_init_done, __ATOMIC_CONSUME) == 0) {
			pthread_cond_wait(&runtime_init_cond, &runtime_init_mu);
		}

		// The key and x_cgo_pthread_key_created are for the whole program,
		// whereas the specific and destructor is per thread.
		if (x_cgo_pthread_key_created == 0 && pthread_key_create(&pthread_g, pthread_key_destructor) == 0) {
			x_cgo_pthread_key_created = 1;
		}

		// TODO(iant): For the case of a new C thread calling into Go, such
		// as when using -buildmode=c-archive, we know that Go runtime
		// initialization is complete but we do not know that all Go init
		// functions have been run. We should not fetch cgo_context_function
		// until they have been, because that is where a call to
		// SetCgoTraceback is likely to occur. We are going to wait for Go
		// initialization to be complete anyhow, later, by waiting for
		// main_init_done to be closed in cgocallbackg1. We should wait here
		// instead. See also issue #15943.
		pfn = __atomic_load_n(&cgo_context_function, __ATOMIC_CONSUME);

		__atomic_store_n(&runtime_init_done, done, __ATOMIC_RELEASE);
		pthread_mutex_unlock(&runtime_init_mu);
	}

	if (pfn != nil) {
		struct cgoContextArg arg;

		arg.Context = 0;
		(*pfn)(&arg);
		return arg.Context;
	}
	return 0;
}

// Store the g into a thread-specific value associated with the pthread key pthread_g.
// And pthread_key_destructor will dropm when the thread is exiting.
void x_cgo_bindm(void* g) {
	// We assume this will always succeed, otherwise, there might be extra M leaking,
	// when a C thread exits after a cgo call.
	// We only invoke this function once per thread in runtime.needAndBindM,
	// and the next calls just reuse the bound m.
	pthread_setspecific(pthread_g, g);
}

void (* _cgo_bindm)(void*) = x_cgo_bindm;

void
x_cgo_notify_runtime_init_done(void* dummy __attribute__ ((unused))) {
	pthread_mutex_lock(&runtime_init_mu);
	__atomic_store_n(&runtime_init_done, 1, __ATOMIC_RELEASE);
	pthread_cond_broadcast(&runtime_init_cond);
	pthread_mutex_unlock(&runtime_init_mu);
}

// Sets the traceback, context, and symbolizer functions. Called from
// runtime.SetCgoTraceback.
void x_cgo_set_traceback_functions(struct cgoSetTracebackFunctionsArg* arg) {
	__atomic_store_n(&cgo_traceback_function, arg->Traceback, __ATOMIC_RELEASE);
	__atomic_store_n(&cgo_context_function, arg->Context, __ATOMIC_RELEASE);
	__atomic_store_n(&cgo_symbolizer_function, arg->Symbolizer, __ATOMIC_RELEASE);
}

// Gets the traceback function to call to trace C calls.
void (*(_cgo_get_traceback_function(void)))(struct cgoTracebackArg*) {
	return __atomic_load_n(&cgo_traceback_function, __ATOMIC_CONSUME);
}

// Call the traceback function registered with x_cgo_set_traceback_functions.
//
// The traceback function is an arbitrary user C function which may be built
// with TSAN, and thus must be wrapped with TSAN acquire/release calls. For
// normal cgo calls, cmd/cgo automatically inserts TSAN acquire/release calls.
// Since the traceback, context, and symbolizer functions are registered at
// startup and called via the runtime, they do not get automatic TSAN
// acquire/release calls.
//
// The only purpose of this wrapper is to perform TSAN acquire/release.
// Alternatively, if the runtime arranged to safely call TSAN acquire/release,
// it could perform the call directly.
void x_cgo_call_traceback_function(struct cgoTracebackArg* arg) {
	void (*pfn)(struct cgoTracebackArg*);

	pfn = _cgo_get_traceback_function();
	if (pfn == nil) {
		return;
	}

	_cgo_tsan_acquire();
	(*pfn)(arg);
	_cgo_tsan_release();
}

// Gets the context function to call to record the traceback context
// when calling a Go function from C code.
void (*(_cgo_get_context_function(void)))(struct cgoContextArg*) {
	return __atomic_load_n(&cgo_context_function, __ATOMIC_CONSUME);
}

// Gets the symbolizer function to call to symbolize C frames.
void (*(_cgo_get_symbolizer_function(void)))(struct cgoSymbolizerArg*) {
	return __atomic_load_n(&cgo_symbolizer_function, __ATOMIC_CONSUME);
}

// Call the symbolizer function registered with x_cgo_set_traceback_functions.
//
// See comment on x_cgo_call_traceback_function.
void x_cgo_call_symbolizer_function(struct cgoSymbolizerArg* arg) {
	void (*pfn)(struct cgoSymbolizerArg*);

	pfn = _cgo_get_symbolizer_function();
	if (pfn == nil) {
		return;
	}

	_cgo_tsan_acquire();
	(*pfn)(arg);
	_cgo_tsan_release();
}

static void
pthread_key_destructor(void* g) {
	if (x_crosscall2_ptr != NULL) {
		// fn == NULL means dropm.
		// We restore g by using the stored g, before dropm in runtime.cgocallback,
		// since the g stored in the TLS by Go might be cleared in some platforms,
		// before this destructor invoked.
		x_crosscall2_ptr(NULL, g, 0, 0);
	}
}

void
x_cgo_thread_start(ThreadStart *arg)
{
	ThreadStart *ts;

	/* Make our own copy that can persist after we return. */
	ts = malloc(sizeof *ts);
	if(ts == nil) {
		fprintf(stderr, "runtime/cgo: out of memory in thread_start\n");
		abort();
	}
	*ts = *arg;

	_cgo_sys_thread_start(ts);	/* OS-dependent half */
}

void (* _cgo_thread_start)(ThreadStart*) = x_cgo_thread_start;