Make it easy to reproduce the race

b1d1f0f5 · Silas Boyd-Wickizer · 2f894520 · b1d1f0f5 · b1d1f0f5 · b1d1f0f5
--- a/include/proc.hh
+++ b/include/proc.hh
@@ -44,6 +44,8 @@ typedef enum procstate {
  ZOMBIE 
 } procstate_t;;

+#define PROC_MAGIC 0xfeedfacedeadd00dULL
+
 // Per-process state
 struct proc : public rcu_freed, public sched_link {
  struct vmap *vmap;           // va -> vma
@@ -85,6 +87,7 @@ struct proc : public rcu_freed, public sched_link {

  std::atomic<int> exception_inuse;
  u8 exception_buf[256];
+  u64 magic;

  static proc* alloc();
  void         set_state(procstate_t s);

--- a/kernel/gc.cc
+++ b/kernel/gc.cc
@@ -136,6 +136,9 @@ gc_delayfreelist(void)
  xnspid->enumerate([&min](u32, proc *p)->bool{
      // Some threads may never call begin/end_epoch(), and never update
      // p->epoch, so gc_thread does it for them.
+      if (p->magic != PROC_MAGIC)
+        panic("gc_delayfreelist: no magic %lx\n", p->magic);
+
      u64 x = p->epoch.load();
      if (!(x & 0xff)) {
        cmpxch(&p->epoch, x, global_epoch.load() << 8);

--- a/kernel/proc.cc
+++ b/kernel/proc.cc
@@ -42,7 +42,7 @@ proc::proc(int npid) :
  ftable(0), cwd(0), tsc(0), curcycles(0), cpuid(0), epoch(0),
  cpu_pin(0), oncv(0), cv_wakeup(0),
  user_fs_(0), unmap_tlbreq_(0), in_exec_(0), uaccess_(0),
-  exception_inuse(0), state_(EMBRYO)
+  exception_inuse(0), magic(PROC_MAGIC), state_(EMBRYO)
 {
  snprintf(lockname, sizeof(lockname), "cv:proc:%d", pid);
  initlock(&lock, lockname+3, LOCKSTAT_PROC);
@@ -57,6 +57,7 @@ proc::proc(int npid) :

 proc::~proc(void)
 {
+  magic = 0;
  if (kstack)
    ksfree(slab_stack, kstack);
  destroylock(&lock);
@@ -410,6 +411,9 @@ finishproc(struct proc *p)
  p->name[0] = 0;
  p->killed = 0;
  freeproc(p);
+#if DEBUG
+  gc_wakeup();
+#endif
 }

 // Wait for a child process to exit and return its pid.
@@ -433,15 +437,15 @@ wait(void)
 	  SLIST_REMOVE(&myproc()->childq, p, proc, child_next);
 	  release(&myproc()->lock);

+          if (!xnspid->remove(pid, &p))
+            panic("wait: ns_remove");
+
          cwork *w = new cwork();
          assert(w);
          w->rip = (void*) finishproc;
          w->arg0 = p;
          assert(wqcrit_push(w, p->run_cpuid_) >= 0);

-          if (!xnspid->remove(pid, &p))
-            panic("wait: ns_remove");
-
 	  return pid;
 	}
 	release(&p->lock);