runtime: add GODEBUG=gccheckmark=0/1

Previously, gccheckmark could only be enabled or disabled by calling
runtime.GCcheckmarkenable/GCcheckmarkdisable.  This was a necessary
hack because GODEBUG was broken.

Now that GODEBUG works again, move control over gccheckmark to a
GODEBUG variable and remove these runtime functions.  Currently,
gccheckmark is enabled by default (and will probably remain so for
much of the 1.5 development cycle).

Change-Id: I2bc6f30c21b795264edf7dbb6bd7354b050673ab
Reviewed-on: https://go-review.googlesource.com/2603
Reviewed-by: Rick Hudson <rlh@golang.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/runtime/extern.go b/src/runtime/extern.go
index f295b9b12c..58acbb3788 100644
--- a/src/runtime/extern.go
+++ b/src/runtime/extern.go
@@ -66,6 +66,12 @@ a comma-separated list of name=val pairs. Supported names are:
 	problem with allocfreetrace=1 in order to understand the type
 	of the badly updated word.
 
+	gccheckmark: setting gccheckmark=1 enables verification of the
+	garbage collector's concurrent mark phase by performing a
+	second mark pass while the world is stopped.  If the second
+	pass finds a reachable object that was not found by concurrent
+	mark, the garbage collector will panic.
+
 The GOMAXPROCS variable limits the number of operating system threads that
 can execute user-level Go code simultaneously. There is no limit to the number of threads
 that can be blocked in system calls on behalf of Go code; those do not count against