recursive read rcu sections; optional rcu write lock

e6958239 · Nickolai Zeldovich · a1845865 · e6958239 · e6958239
--- a/proc.h
+++ b/proc.h
@@ -91,6 +91,7 @@ struct proc {
  SLIST_ENTRY(proc) child_next;
  struct condvar cv;
  uint epoch;
+  uint rcu_read_depth;
  char lockname[16];
 };

--- a/rcu.c
+++ b/rcu.c
@@ -97,30 +97,33 @@ rcu_delayed(void *e, void (*dofree)(void *))
 void
 rcu_begin_read(void)
 {
-  proc->epoch = global_epoch;
+  if (proc->rcu_read_depth++ == 0)
+    proc->epoch = global_epoch;
  __sync_synchronize();
 }
 void
 rcu_end_read(void)
 {
-  proc->epoch = INF;
+  if (--proc->rcu_read_depth == 0)
+    proc->epoch = INF;
 }
 void
 rcu_begin_write(struct spinlock *l)
 {
-  acquire(l);
+  rcu_begin_read();
+  if (l) acquire(l);
+  __sync_synchronize();
 }
-// XXX if a process never calls rcu_end_write() we have a problem; run
-// rcu_gc from a kernel thread periodically?
 void
 rcu_end_write(struct spinlock *l)
 {
-  // for other data structures using rcu, use atomic add:
+  // global_epoch can be bumped anywhere; this seems as good a place as any
  __sync_fetch_and_add(&global_epoch, 1);
-  release(l);
-  // rcu_gc();
+  if (l) release(l);
+  rcu_end_read();
 }