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, 13 insertions, 3 deletions

diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 645083db07..adfdaced18 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -211,14 +211,24 @@ func markroot(gcw *gcWork, i uint32) {
 				userG.waitreason = waitReasonGarbageCollectionScan
 			}
 
-			// TODO: scang blocks until gp's stack has
-			// been scanned, which may take a while for
+			// TODO: suspendG blocks (and spins) until gp
+			// stops, which may take a while for
 			// running goroutines. Consider doing this in
 			// two phases where the first is non-blocking:
 			// we scan the stacks we can and ask running
 			// goroutines to scan themselves; and the
 			// second blocks.
-			scang(gp, gcw)
+			stopped := suspendG(gp)
+			if stopped.dead {
+				gp.gcscandone = true
+				return
+			}
+			if gp.gcscandone {
+				throw("g already scanned")
+			}
+			scanstack(gp, gcw)
+			gp.gcscandone = true
+			resumeG(stopped)
 
 			if selfScan {
 				casgstatus(userG, _Gwaiting, _Grunning)