runtime: add general suspendG/resumeG

Currently, the process of suspending a goroutine is tied to stack
scanning. In preparation for non-cooperative preemption, this CL
abstracts this into general purpose suspendG/resumeG functions.

suspendG and resumeG closely follow the existing scang and restartg
functions with one exception: the addition of a _Gpreempted status.
Currently, preemption tasks (stack scanning) are carried out by the
target goroutine if it's in _Grunning. In this new approach, the task
is always carried out by the goroutine that called suspendG. Thus, we
need a reliable way to drive the target goroutine out of _Grunning
until the requesting goroutine is ready to resume it. The new
_Gpreempted state provides the handshake: when a runnable goroutine
responds to a preemption request, it now parks itself and enters
_Gpreempted. The requesting goroutine races to put it in _Gwaiting,
which gives it ownership, but also the responsibility to start it
again.

This CL adds several TODOs about improving the synchronization on the
G status. The existing code already has these problems; we're just
taking note of them.

The next CL will remove the now-dead scang and preemptscan.

For #10958, #24543.

Change-Id: I16dbf87bea9d50399cc86719c156f48e67198f16
Reviewed-on: https://go-review.googlesource.com/c/go/+/201137
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/runtime/traceback.go b/src/runtime/traceback.go
index 0e4b75a7e6..9be7d739d1 100644
--- a/src/runtime/traceback.go
+++ b/src/runtime/traceback.go
@@ -860,6 +860,7 @@ var gStatusStrings = [...]string{
 	_Gwaiting:   "waiting",
 	_Gdead:      "dead",
 	_Gcopystack: "copystack",
+	_Gpreempted: "preempted",
 }
 
 func goroutineheader(gp *g) {