runtime: handle m0 padding better

The SpinbitMutex experiment requires m structs other than m0
to be allocated in 2048-byte size class, by adding padding.
Do the calculation more explicitly, to avoid future CLs like CL 653335.

Change-Id: I83ae1e86ef3711ab65441f4e487f94b9e1429029
Reviewed-on: https://go-review.googlesource.com/c/go/+/654595
Reviewed-by: Rhys Hiltner <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>

Author: rsc

src/runtime/asan.go

@@ -26,6 +26,7 @@ func ASanWrite(addr unsafe.Pointer, len int) {
 
 // Private interface for the runtime.
 const asanenabled = true
+const asanenabledBit = 1
 
 // asan{read,write} are nosplit because they may be called between
 // fork and exec, when the stack must not grow. See issue #50391.

src/runtime/asan0.go

@@ -13,6 +13,7 @@ import (
 )
 
 const asanenabled = false
+const asanenabledBit = 0
 
 // Because asanenabled is false, none of these functions should be called.
 

src/runtime/lock_spinbit.go

@@ -90,7 +90,7 @@ type mWaitList struct {
 
 // lockVerifyMSize confirms that we can recreate the low bits of the M pointer.
 func lockVerifyMSize() {
-	size := roundupsize(unsafe.Sizeof(m{}), false) + mallocHeaderSize
+	size := roundupsize(unsafe.Sizeof(mPadded{}), false) + mallocHeaderSize
 	if size&mutexMMask != 0 {
 		print("M structure uses sizeclass ", size, "/", hex(size), " bytes; ",
 			"incompatible with mutex flag mask ", hex(mutexMMask), "\n")

src/runtime/proc.go

@@ -2256,7 +2256,7 @@ func allocm(pp *p, fn func(), id int64) *m {
 		unlock(&sched.lock)
 	}
 
-	mp := new(m)
+	mp := &new(mPadded).m
 	mp.mstartfn = fn
 	mcommoninit(mp, id)
 

src/runtime/runtime2.go

@@ -619,13 +619,18 @@ type m struct {
 	// Up to 10 locks held by this m, maintained by the lock ranking code.
 	locksHeldLen int
 	locksHeld    [10]heldLockInfo
+}
+
+const mRedZoneSize = (16 << 3) * asanenabledBit // redZoneSize(2048)
+
+type mPadded struct {
+	m
 
 	// Size the runtime.m structure so it fits in the 2048-byte size class, and
 	// not in the next-smallest (1792-byte) size class. That leaves the 11 low
 	// bits of muintptr values available for flags, as required for
 	// GOEXPERIMENT=spinbitmutex.
-	_ [goexperiment.SpinbitMutexInt * 64 * goarch.PtrSize / 8]byte
-	_ [goexperiment.SpinbitMutexInt * 700 * (2 - goarch.PtrSize/4)]byte
+	_ [goexperiment.SpinbitMutexInt * (2048 - mallocHeaderSize - mRedZoneSize - unsafe.Sizeof(m{}))]byte
 }
 
 type p struct {