tricky case for inodes, which are both rcu and long-term-refcounted

7663c33b · Nickolai Zeldovich · a8e17d76 · 7663c33b
--- a/fs.c
+++ b/fs.c
@@ -229,27 +229,41 @@ iget(uint dev, uint inum)

 retry:
  // Try for cached inode.
+  rcu_begin_read();
  ip = ns_lookup(ins, inum);	// XXX ignore dev
  if (ip) {
    if (ip->dev != dev) panic("iget dev mismatch");
-    acquire(&ip->lock);
+    // tricky: first bump ref, then check free flag
    __sync_fetch_and_add(&ip->ref, 1);
    if (ip->flags & I_FREE) {
-      release(&ip->lock);
+      rcu_end_read();
+      __sync_sub_and_fetch(&ip->ref, 1);
      goto retry;
    }
-
-    while((ip->flags & I_VALID) == 0)
-      cv_sleep(&ip->cv, &ip->lock);
-    release(&ip->lock);
+    rcu_end_read();
+    if (!(ip->flags & I_VALID)) {
+      acquire(&ip->lock);
+      while((ip->flags & I_VALID) == 0)
+	cv_sleep(&ip->cv, &ip->lock);
+      release(&ip->lock);
+    }
    return ip;
  }
+  rcu_end_read();

  // Allocate fresh inode cache slot.
+ retry_evict:
+  (void) 0;
  struct inode *victim = ns_enumerate(ins, evict);
  if (!victim)
    panic("iget out of space");
+  // tricky: first flag as free, then check refcnt, then remove from ns
  victim->flags |= I_FREE;
+  if (victim->ref > 0) {
+    victim->flags &= ~(I_FREE);
+    release(&victim->lock);
+    goto retry_evict;
+  }
  release(&victim->lock);
  ns_remove(ins, victim->inum, victim);
  rcu_delayed(victim, kmfree);