chore: use atomics instead of artisinal assembly

Author: mikesmitty

src/runtime/runtime_rp2350.go

@@ -3,8 +3,8 @@
 package runtime
 
 import (
-	"device/arm"
 	"device/rp"
+	"sync/atomic"
 )
 
 const (
@@ -18,57 +18,22 @@ const (
 
 // Due to hardware errata RP2350-E2 spinlocks are emulated in software as in pico-sdk.
 type spinLock struct {
-	// state field must be first field so its address is the same as the struct address.
-	state  uint8
-	id     uint8
-	uint16 // Padding to prevent false sharing
+	atomic.Uint32
 }
 
 func (l *spinLock) Lock() {
-	// Original reference:
-	// https://github.com/raspberrypi/pico-sdk/blob/2.2.0/src/rp2_common/hardware_sync_spin_lock/include/hardware/sync/spin_lock.h#L112
-
-	// r0 is automatically filled with the pointer value "l" here.
-	// We create a variable to permit access to the state byte (l.state) and
-	// avoid a memory fault when accessing it in assembly.
-	state := &l.state
-	_ = state
-
-	// Set the loop start point.
-	arm.Asm("1:")
-	// Exclusively load (lock) the state byte and put its value in r2.
-	arm.Asm("ldaexb r2, [r0]")
-	// Set the r1 register to '1' for later use.
-	arm.Asm("movs r1, #1")
-	// Check if the lock was already taken (r2 != 0).
-	arm.Asm("cmp r2, #0")
-	// Jump back to the loop start ("1:") if the lock is already held.
-	arm.Asm("bne 1b")
-
-	// Attempt to store '1' into the lock state byte.
-	// The return code (0 for success, 1 for failure) is placed in r2.
-	arm.Asm("strexb r2, r1, [r0]")
-	// Check if the result was successful (r2 == 0).
-	arm.Asm("cmp r2, #0")
-	// Jump back to the loop start ("1:") if the lock was not acquired.
-	arm.Asm("bne 1b")
-
-	// Memory barrier to ensure everyone knows we're holding the lock now.
-	arm.Asm("dmb")
+	// Try to replace 0 with 1. Once we succeed, the lock has been acquired.
+	for !l.Uint32.CompareAndSwap(0, 1) {
+		spinLoopWait()
+	}
 }
 
 func (l *spinLock) Unlock() {
-	// Original reference:
-	// https://github.com/raspberrypi/pico-sdk/blob/2.2.0/src/rp2_common/hardware_sync_spin_lock/include/hardware/sync/spin_lock.h#L197
+	// Safety check: the spinlock should have been locked.
+	if schedulerAsserts && l.Uint32.Load() != 1 {
+		runtimePanic("unlock of unlocked spinlock")
+	}
 
-	// r0 is automatically filled with the pointer value l here.
-	// We create a variable to permit access to the state byte (l.state) and
-	// avoid a memory fault when accessing it in assembly.
-	state := &l.state
-	_ = state
-	// Fill r1 with 0 and store it to the state byte address in r0. stlb
-	// requires a register as a source so we can't use a literal 0 directly.
-	arm.Asm("movs r1, #0")
-	// Release the pseudo-spinlock by writing 0 to and releasing l.state.
-	arm.Asm("stlb r1, [r0]")
+	// Unlock the lock. Simply write 0, because we already know it is locked.
+	l.Uint32.Store(0)
 }