libgo patch committed: Drop G before changing status

[Ian Lance Taylor]

This libgo patch by Cherry Zhang changes the runtime package to call
dropg before changing the  g status to _Grunnable/_Gwaiting.
Currently, we dropg (which clears gp.m) after we CAS the g status to
_Grunnable or _Gwaiting.  Immediately after CASing the g status,
another thread may CAS it to _Gscan status and scan its stack.  With
precise stack scan, it accesses gp.m in order to switch to g and back
(in doscanstackswitch).  This races with dropg.  If doscanstackswitch
reads gp.m, then dropg runs, when we restore the m at the end of the
scan it will set to a stale value.  Worse, if dropg runs after
doscanstackswitch sets the new m, gp will be running with a nil m.

To fix this, we do dropg before CAS g status to _Grunnable or
_Gwaiting.  We can do this safely if we are CASing from _Grunning, as
we own the g when it is in _Grunning.  There is one case where we CAS
from _Gsyscall to _Grunnable.  It is not safe to dropg when it is in
_Gsyscall, as precise stack scan needs to read gp.m in order to signal
the m.  So we need to introduce a transient state, _Gexitingsyscall,
between _Gsyscall and _Grunnable, where the GC should not scan its
stack.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian
-------------- next part --------------
Index: gcc/go/gofrontend/MERGE
===================================================================
--- gcc/go/gofrontend/MERGE	(revision 267989)
+++ gcc/go/gofrontend/MERGE	(working copy)
@@ -1,4 +1,4 @@
-ee94431c133a90ca5c3c5ebbebcb019c60258dac
+d6576c83016d856217758c06d945bfc363ffb817
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
Index: libgo/go/runtime/proc.go
===================================================================
--- libgo/go/runtime/proc.go	(revision 267941)
+++ libgo/go/runtime/proc.go	(working copy)
@@ -956,6 +956,10 @@ loop:
 				break loop
 			}
 
+		case _Gexitingsyscall:
+			// This is a transient state during which we should not scan its stack.
+			// Try again.
+
 		case _Gscanwaiting:
 			// newstack is doing a scan for us right now. Wait.
 
@@ -2635,8 +2639,8 @@ func park_m(gp *g) {
 		traceGoPark(_g_.m.waittraceev, _g_.m.waittraceskip)
 	}
 
-	casgstatus(gp, _Grunning, _Gwaiting)
 	dropg()
+	casgstatus(gp, _Grunning, _Gwaiting)
 
 	if _g_.m.waitunlockf != nil {
 		fn := *(*func(*g, unsafe.Pointer) bool)(unsafe.Pointer(&_g_.m.waitunlockf))
@@ -2660,8 +2664,8 @@ func goschedImpl(gp *g) {
 		dumpgstatus(gp)
 		throw("bad g status")
 	}
-	casgstatus(gp, _Grunning, _Grunnable)
 	dropg()
+	casgstatus(gp, _Grunning, _Grunnable)
 	lock(&sched.lock)
 	globrunqput(gp)
 	unlock(&sched.lock)
@@ -3054,8 +3058,9 @@ func exitsyscallfast_pidle() bool {
 func exitsyscall0(gp *g) {
 	_g_ := getg()
 
-	casgstatus(gp, _Gsyscall, _Grunnable)
+	casgstatus(gp, _Gsyscall, _Gexitingsyscall)
 	dropg()
+	casgstatus(gp, _Gexitingsyscall, _Grunnable)
 	lock(&sched.lock)
 	_p_ := pidleget()
 	if _p_ == nil {
Index: libgo/go/runtime/runtime2.go
===================================================================
--- libgo/go/runtime/runtime2.go	(revision 267941)
+++ libgo/go/runtime/runtime2.go	(working copy)
@@ -70,6 +70,12 @@ const (
 	// stack is owned by the goroutine that put it in _Gcopystack.
 	_Gcopystack // 8
 
+	// _Gexitingsyscall means this goroutine is exiting from a
+	// system call. This is like _Gsyscall, but the GC should not
+	// scan its stack. Currently this is only used in exitsyscall0
+	// as a transient state when it drops the G.
+	_Gexitingsyscall // 9
+
 	// _Gscan combined with one of the above states other than
 	// _Grunning indicates that GC is scanning the stack. The
 	// goroutine is not executing user code and the stack is owned
Index: libgo/go/runtime/traceback_gccgo.go
===================================================================
--- libgo/go/runtime/traceback_gccgo.go	(revision 267941)
+++ libgo/go/runtime/traceback_gccgo.go	(working copy)
@@ -122,13 +122,14 @@ func isExportedRuntime(name string) bool
 }
 
 var gStatusStrings = [...]string{
-	_Gidle:      "idle",
-	_Grunnable:  "runnable",
-	_Grunning:   "running",
-	_Gsyscall:   "syscall",
-	_Gwaiting:   "waiting",
-	_Gdead:      "dead",
-	_Gcopystack: "copystack",
+	_Gidle:           "idle",
+	_Grunnable:       "runnable",
+	_Grunning:        "running",
+	_Gsyscall:        "syscall",
+	_Gwaiting:        "waiting",
+	_Gdead:           "dead",
+	_Gcopystack:      "copystack",
+	_Gexitingsyscall: "exiting syscall",
 }
 
 func goroutineheader(gp *g) {