runtime: use signals to preempt Gs for suspendG

This adds support for pausing a running G by sending a signal to its
M.

The main complication is that we want to target a G, but can only send
a signal to an M. Hence, the protocol we use is to simply mark the G
for preemption (which we already do) and send the M a "wake up and
look around" signal. The signal checks if it's running a G with a
preemption request and stops it if so in the same way that stack check
preemptions stop Gs. Since the preemption may fail (the G could be
moved or the signal could arrive at an unsafe point), we keep a count
of the number of received preemption signals. This lets stopG detect
if its request failed and should be retried without an explicit
channel back to suspendG.

For #10958, #24543.

Change-Id: I3e1538d5ea5200aeb434374abb5d5fdc56107e53
Reviewed-on: https://go-review.googlesource.com/c/go/+/201760
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 0087408a72..10b525b2bc 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -196,7 +196,7 @@ func markroot(gcw *gcWork, i uint32) {
 			gp.waitsince = work.tstart
 		}
 
-		// scang must be done on the system stack in case
+		// scanstack must be done on the system stack in case
 		// we're trying to scan our own stack.
 		systemstack(func() {
 			// If this is a self-scan, put the user G in
@@ -716,6 +716,10 @@ func scanstack(gp *g, gcw *gcWork) {
 		println("stack trace goroutine", gp.goid)
 	}
 
+	if debugScanConservative && gp.asyncSafePoint {
+		print("scanning async preempted goroutine ", gp.goid, " stack [", hex(gp.stack.lo), ",", hex(gp.stack.hi), ")\n")
+	}
+
 	// Scan the saved context register. This is effectively a live
 	// register that gets moved back and forth between the
 	// register and sched.ctxt without a write barrier.