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, 16 insertions, 5 deletions

diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index c5023027be..7eac58eb2c 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -78,6 +78,13 @@ const (
 	// stack is owned by the goroutine that put it in _Gcopystack.
 	_Gcopystack // 8
 
+	// _Gpreempted means this goroutine stopped itself for a
+	// suspendG preemption. It is like _Gwaiting, but nothing is
+	// yet responsible for ready()ing it. Some suspendG must CAS
+	// the status to _Gwaiting to take responsibility for
+	// ready()ing this G.
+	_Gpreempted // 9
+
 	// _Gscan combined with one of the above states other than
 	// _Grunning indicates that GC is scanning the stack. The
 	// goroutine is not executing user code and the stack is owned
@@ -89,11 +96,12 @@ const (
 	//
 	// atomicstatus&~Gscan gives the state the goroutine will
 	// return to when the scan completes.
-	_Gscan         = 0x1000
-	_Gscanrunnable = _Gscan + _Grunnable // 0x1001
-	_Gscanrunning  = _Gscan + _Grunning  // 0x1002
-	_Gscansyscall  = _Gscan + _Gsyscall  // 0x1003
-	_Gscanwaiting  = _Gscan + _Gwaiting  // 0x1004
+	_Gscan          = 0x1000
+	_Gscanrunnable  = _Gscan + _Grunnable  // 0x1001
+	_Gscanrunning   = _Gscan + _Grunning   // 0x1002
+	_Gscansyscall   = _Gscan + _Gsyscall   // 0x1003
+	_Gscanwaiting   = _Gscan + _Gwaiting   // 0x1004
+	_Gscanpreempted = _Gscan + _Gpreempted // 0x1009
 )
 
 const (
@@ -411,6 +419,7 @@ type g struct {
 	waitsince      int64      // approx time when the g become blocked
 	waitreason     waitReason // if status==Gwaiting
 	preempt        bool       // preemption signal, duplicates stackguard0 = stackpreempt
+	preemptStop    bool       // transition to _Gpreempted on preemption; otherwise, just deschedule
 	paniconfault   bool       // panic (instead of crash) on unexpected fault address
 	preemptscan    bool       // preempted g does scan for gc
 	gcscandone     bool       // g has scanned stack; protected by _Gscan bit in status
@@ -906,6 +915,7 @@ const (
 	waitReasonTraceReaderBlocked                      // "trace reader (blocked)"
 	waitReasonWaitForGCCycle                          // "wait for GC cycle"
 	waitReasonGCWorkerIdle                            // "GC worker (idle)"
+	waitReasonPreempted                               // "preempted"
 )
 
 var waitReasonStrings = [...]string{
@@ -934,6 +944,7 @@ var waitReasonStrings = [...]string{
 	waitReasonTraceReaderBlocked:    "trace reader (blocked)",
 	waitReasonWaitForGCCycle:        "wait for GC cycle",
 	waitReasonGCWorkerIdle:          "GC worker (idle)",
+	waitReasonPreempted:             "preempted",
 }
 
 func (w waitReason) String() string {