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 00787ade04ac447b9348b222a895504a0fa7e507..cfd24e06fb684282e9d6742eb93c0081f18a8784 100644 (file)
--- 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.