runtime: remove write barriers from newstack, gogo

Currently, newstack and gogo have write barriers for maintaining the
context register saved in g.sched.ctxt. This is troublesome, because
newstack can be called from go:nowritebarrierrec places that can't
allow write barriers. It happens to be benign because g.sched.ctxt
will always be nil on entry to newstack *and* it so happens the
incoming ctxt will also always be nil in these contexts (I
think/hope), but this is playing with fire. It's also desirable to
mark newstack go:nowritebarrierrec to prevent any other, non-benign
write barriers from creeping in, but we can't do that right now
because of this one write barrier.

Fix all of this by observing that g.sched.ctxt is really just a saved
live pointer register. Hence, we can shade it when we scan g's stack
and otherwise move it back and forth between the actual context
register and g.sched.ctxt without write barriers. This means we can
save it in morestack along with all of the other g.sched, eliminate
the save from newstack along with its troublesome write barrier, and
eliminate the shenanigans in gogo to invoke the write barrier when
restoring it.

Once we've done all of this, we can mark newstack
go:nowritebarrierrec.

Fixes #22385.
For #22460.

Change-Id: I43c24958e3f6785b53c1350e1e83c2844e0d1522
Reviewed-on: https://go-review.googlesource.com/72553
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 10 insertions, 8 deletions

diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index ca796169fe..a79faba8ce 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -254,17 +254,19 @@ type gobuf struct {
 	// The offsets of sp, pc, and g are known to (hard-coded in) libmach.
 	//
 	// ctxt is unusual with respect to GC: it may be a
-	// heap-allocated funcval so write require a write barrier,
-	// but gobuf needs to be cleared from assembly. We take
-	// advantage of the fact that the only path that uses a
-	// non-nil ctxt is morestack. As a result, gogo is the only
-	// place where it may not already be nil, so gogo uses an
-	// explicit write barrier. Everywhere else that resets the
-	// gobuf asserts that ctxt is already nil.
+	// heap-allocated funcval, so GC needs to track it, but it
+	// needs to be set and cleared from assembly, where it's
+	// difficult to have write barriers. However, ctxt is really a
+	// saved, live register, and we only ever exchange it between
+	// the real register and the gobuf. Hence, we treat it as a
+	// root during stack scanning, which means assembly that saves
+	// and restores it doesn't need write barriers. It's still
+	// typed as a pointer so that any other writes from Go get
+	// write barriers.
 	sp   uintptr
 	pc   uintptr
 	g    guintptr
-	ctxt unsafe.Pointer // this has to be a pointer so that gc scans it
+	ctxt unsafe.Pointer
 	ret  sys.Uintreg
 	lr   uintptr
 	bp   uintptr // for GOEXPERIMENT=framepointer