From: Austin Clements <austin@google.com>
Date: Thu, 17 Nov 2016 15:48:40 +0000 (-0500)
Subject: runtime: improve diagnostics for "scan missed a g"
X-Git-Tag: go1.8beta1~111
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=05dc6b26ca337b80f9424aa9b78f4b45f6e44c8d;p=gostls13.git

runtime: improve diagnostics for "scan missed a g"

Currently there are no diagnostics for mark root check during marking.
Fix this by printing out the same diagnostics we print during mark
termination.

Also, drop the allglock before throwing. Holding that across a throw
causes a self-deadlock with tracebackothers.

For #16083.

Change-Id: Ib605f3ae0c17e70704b31d8378274cfaa2307dc2
Reviewed-on: https://go-review.googlesource.com/33339
Reviewed-by: Rick Hudson <rlh@golang.org>
---

diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 00787ade04..cfd24e06fb 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -126,26 +126,32 @@ func gcMarkRootCheck() {
 
 	lock(&allglock)
 	// Check that stacks have been scanned.
+	var gp *g
 	if gcphase == _GCmarktermination && debug.gcrescanstacks > 0 {
 		for i := 0; i < len(allgs); i++ {
-			gp := allgs[i]
+			gp = allgs[i]
 			if !(gp.gcscandone && gp.gcscanvalid) && readgstatus(gp) != _Gdead {
-				println("gp", gp, "goid", gp.goid,
-					"status", readgstatus(gp),
-					"gcscandone", gp.gcscandone,
-					"gcscanvalid", gp.gcscanvalid)
-				throw("scan missed a g")
+				goto fail
 			}
 		}
 	} else {
 		for i := 0; i < work.nStackRoots; i++ {
-			gp := allgs[i]
+			gp = allgs[i]
 			if !gp.gcscandone {
-				throw("scan missed a g")
+				goto fail
 			}
 		}
 	}
 	unlock(&allglock)
+	return
+
+fail:
+	println("gp", gp, "goid", gp.goid,
+		"status", readgstatus(gp),
+		"gcscandone", gp.gcscandone,
+		"gcscanvalid", gp.gcscanvalid)
+	unlock(&allglock) // Avoid self-deadlock with traceback.
+	throw("scan missed a g")
 }
 
 // ptrmask for an allocation containing a single pointer.