runtime/cgo: reduce runtime init done check using atomic

Every call from C to Go does acquire a mutex to check whether Go runtime
has been fully initialized. This often does not matter, because the lock
is held only briefly. However, with code that does a lot of parallel
calls from C to Go could cause heavy contention on the mutex.

Since this is an initialization guard, we can double check with atomic
operation to provide a fast path in case the initialization is done.
With this CL, program in #60961 reduces from ~2.7s to ~1.8s.

Fixes #60961

Change-Id: Iba4cabbee3c9bc646e70ef7eb074212ba63fdc04
Reviewed-on: https://go-review.googlesource.com/c/go/+/505455
Reviewed-by: Heschi Kreinick <[email protected]>
Auto-Submit: Cuong Manh Le <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/runtime/cgo/gcc_libinit.c

@@ -41,30 +41,37 @@ x_cgo_sys_thread_create(void* (*func)(void*), void* arg) {
 uintptr_t
 _cgo_wait_runtime_init_done(void) {
  void (*pfn)(struct context_arg*);
+ pfn = __atomic_load_n(&cgo_context_function, __ATOMIC_CONSUME);
 
- pthread_mutex_lock(&runtime_init_mu);
- while (runtime_init_done == 0) {
- pthread_cond_wait(&runtime_init_cond, &runtime_init_mu);
- }
+ int 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;
- }
+  // 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 = cgo_context_function;
 
- pthread_mutex_unlock(&runtime_init_mu);
+ // 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 context_arg arg;
 
@@ -88,27 +95,20 @@ void x_cgo_bindm(void* g) {
 void
 x_cgo_notify_runtime_init_done(void* dummy __attribute__ ((unused))) {
  pthread_mutex_lock(&runtime_init_mu);
- runtime_init_done = 1;
+ __atomic_store_n(&runtime_init_done, 1, __ATOMIC_RELEASE);
  pthread_cond_broadcast(&runtime_init_cond);
  pthread_mutex_unlock(&runtime_init_mu);
 }
 
 // Sets the context function to call to record the traceback context
 // when calling a Go function from C code. Called from runtime.SetCgoTraceback.
 void x_cgo_set_context_function(void (*context)(struct context_arg*)) {
- pthread_mutex_lock(&runtime_init_mu);
- cgo_context_function = context;
- pthread_mutex_unlock(&runtime_init_mu);
+ __atomic_store_n(&cgo_context_function, context, __ATOMIC_RELEASE);
 }
 
 // Gets the context function.
 void (*(_cgo_get_context_function(void)))(struct context_arg*) {
- void (*ret)(struct context_arg*);
-
- pthread_mutex_lock(&runtime_init_mu);
- ret = cgo_context_function;
- pthread_mutex_unlock(&runtime_init_mu);
- return ret;
+ return __atomic_load_n(&cgo_context_function, __ATOMIC_CONSUME);
 }
 
 // _cgo_try_pthread_create retries pthread_create if it fails with