runtime: dropg before CAS 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.
    
    In is a little unfortunate that we have to add another g status.
    We could reuse an existing one (e.g. _Gcopystack), but it is
    clearer and safer to just use a new one, as Austin suggested.
    
    Reviewed-on: https://go-review.googlesource.com/c/158157

From-SVN: r268001

1 files changed, 8 insertions, 7 deletions

diff --git a/libgo/go/runtime/traceback_gccgo.go b/libgo/go/runtime/traceback_gccgo.go
index 7347cea71b2..72a83a5ab2d 100644
--- a/libgo/go/runtime/traceback_gccgo.go
+++ b/libgo/go/runtime/traceback_gccgo.go
@@ -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) {