libgo patch committed: Don't crash if no p in kickoff

[Ian Lance Taylor]

In libgo the kickoff function for g0 can be invoked without a p, for
example from mcall(exitsyscall0) in exitsyscall after exitsyscall has
cleared the p field. The assignment gp.param = nil will invoke a write
barrier.  If gp.param is not already nil, this will require a p. Avoid
the problem for a specific case that is known to be OK: when the value
in gp.param is a *g.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline.

Ian

Index: gcc/go/gofrontend/MERGE
===================================================================
--- gcc/go/gofrontend/MERGE     (revision 249594)
+++ gcc/go/gofrontend/MERGE     (working copy)
@@ -1,4 +1,4 @@
-29c61dc3c5151df5de9362b7882ccf04679df976
+f107cc8bced1939b0083231fc1ea24669ca4832c
 
 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 249577)
+++ libgo/go/runtime/proc.go    (working copy)
@@ -1097,7 +1097,25 @@ func kickoff() {
        fv := gp.entry
        param := gp.param
        gp.entry = nil
+
+       // When running on the g0 stack we can wind up here without a p,
+       // for example from mcall(exitsyscall0) in exitsyscall.
+       // Setting gp.param = nil will call a write barrier, and if
+       // there is no p that write barrier will crash. When called from
+       // mcall the gp.param value will be a *g, which we don't need to
+       // shade since we know it will be kept alive elsewhere. In that
+       // case clear the field using uintptr so that the write barrier
+       // does nothing.
+       if gp.m.p == 0 {
+               if gp == gp.m.g0 && gp.param == unsafe.Pointer(gp.m.curg) {
+                       *(*uintptr)(unsafe.Pointer(&gp.param)) = 0
+               } else {
+                       throw("no p in kickoff")
+               }
+       }
+
        gp.param = nil
+
        fv(param)
        goexit1()
 }