internal/llsync: implement primitives for preemptive scheduling

Author: aykevl

src/internal/llsync/atomic-cooperative.go

@@ -1,3 +1,5 @@
+//go:build !scheduler.threads
+
 package llsync
 
 // Atomics implementation for cooperative systems. The atomic types here aren't

src/internal/llsync/atomic-preemptive.go

@@ -0,0 +1,14 @@
+//go:build scheduler.threads
+
+package llsync
+
+// Atomics implementation for non-cooperative systems (multithreaded, etc).
+// These atomic types use real atomic instructions.
+
+import "sync/atomic"
+
+type (
+	Uintptr = atomic.Uintptr
+	Uint32  = atomic.Uint32
+	Uint64  = atomic.Uint64
+)

src/internal/llsync/futex-cooperative.go

@@ -1,3 +1,5 @@
+//go:build !scheduler.threads
+
 package llsync
 
 import (

src/internal/llsync/futex-preemptive.go

@@ -0,0 +1,12 @@
+//go:build scheduler.threads
+
+package llsync
+
+// A futex is a way for userspace to wait with the pointer as the key, and for
+// another thread to wake one or all waiting threads keyed on the same pointer.
+//
+// A futex does not change the underlying value, it only reads it before to prevent
+// lost wake-ups.
+type Futex struct {
+	Uint32
+}

src/internal/llsync/futex_linux.c

@@ -0,0 +1,20 @@
+//go:build none
+
+// This file is manually included, to avoid CGo which would cause a circular
+// import.
+
+#include <stdint.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#define FUTEX_WAIT    0
+#define FUTEX_WAKE    1
+#define FUTEX_PRIVATE 128
+
+void tinygo_futex_wait(uint32_t *addr, uint32_t cmp) {
+    syscall(SYS_futex, addr, FUTEX_WAIT|FUTEX_PRIVATE, cmp, NULL, NULL, 0);
+}
+
+void tinygo_futex_wake(uint32_t *addr, uint32_t num) {
+    syscall(SYS_futex, addr, FUTEX_WAKE|FUTEX_PRIVATE, num, NULL, NULL, 0);
+}

src/internal/llsync/futex_linux.go

@@ -0,0 +1,49 @@
+//go:build scheduler.threads
+
+package llsync
+
+import (
+	"unsafe"
+)
+
+// Atomically check for cmp to still be equal to the futex value and if so, go
+// to sleep. Return true if we were definitely awoken by a call to Wake or
+// WakeAll, and false if we can't be sure of that.
+func (f *Futex) Wait(cmp uint32) bool {
+	tinygo_futex_wait((*uint32)(unsafe.Pointer(&f.Uint32)), cmp)
+
+	// We *could* detect a zero return value from the futex system call which
+	// would indicate we got awoken by a Wake or WakeAll call. However, this is
+	// what the manual page has to say:
+	//
+	// > Note that a wake-up can also be caused by common futex usage patterns
+	// > in unrelated code that happened to have previously used the futex
+	// > word's memory location (e.g., typical futex-based implementations of
+	// > Pthreads mutexes can cause this under some conditions). Therefore,
+	// > callers should always conservatively assume that a return value of 0
+	// > can mean a spurious wake-up, and use the futex word's value (i.e., the
+	// > user-space synchronization scheme) to decide whether to continue to
+	// > block or not.
+	//
+	// I'm not sure whether we do anything like pthread does, so to be on the
+	// safe side we say we don't know whether the wakeup was spurious or not and
+	// return false.
+	return false
+}
+
+// Wake a single waiter.
+func (f *Futex) Wake() {
+	tinygo_futex_wake((*uint32)(unsafe.Pointer(&f.Uint32)), 1)
+}
+
+// Wake all waiters.
+func (f *Futex) WakeAll() {
+	const maxInt32 = 0x7fff_ffff
+	tinygo_futex_wake((*uint32)(unsafe.Pointer(&f.Uint32)), maxInt32)
+}
+
+//export tinygo_futex_wait
+func tinygo_futex_wait(addr *uint32, cmp uint32)
+
+//export tinygo_futex_wake
+func tinygo_futex_wake(addr *uint32, num uint32)

src/internal/llsync/plock-cooperative.go

@@ -1,3 +1,5 @@
+//go:build !scheduler.threads
+
 package llsync
 
 // PMutex is a real mutex on systems that can be either preemptive or threaded,

src/internal/llsync/plock-preemptive.go

@@ -0,0 +1,37 @@
+//go:build scheduler.threads
+
+package llsync
+
+// PMutex is a real mutex on systems that can be either preemptive or threaded,
+// and a dummy lock on other (purely cooperative) systems.
+//
+// It is mainly useful for short operations that need a lock when threading may
+// be involved, but which do not need a lock with a purely cooperative
+// scheduler.
+type PMutex struct {
+	futex Futex
+}
+
+func (m *PMutex) Lock() {
+	// Fast path: try to take an uncontended lock.
+	if m.futex.CompareAndSwap(0, 1) {
+		// We obtained the mutex.
+		return
+	}
+
+	// Try to lock the mutex. If it changed from 0 to 2, we took a contended
+	// lock.
+	for m.futex.Swap(2) != 0 {
+		// Wait until we get resumed in Unlock.
+		m.futex.Wait(2)
+	}
+}
+
+func (m *PMutex) Unlock() {
+	if old := m.futex.Swap(0); old == 2 {
+		// Mutex was a contended lock, so we need to wake the next waiter.
+		m.futex.Wake()
+	}
+	// Note: this implementation doesn't check for an unlock of an unlocked
+	// mutex to keep it as lightweight as possible.
+}