tmpfs: use a seperate lock to protect all_nodes

Previously, tmpfs used diskfs_node_refcnt_lock to serialize access to
the all_nodes and some other related global state related to memory
consumption.

Use a separate lock to protect all_nodes, and atomic operations to
access the state related to memory consumption. Adjust the reference
counting accordingly.  Every node in the all_nodes carries a light
reference.  When we are asked to give up that light reference, we
reacquire our lock momentarily to check whether someone else
reacquired a reference through the all_nodes.

* tmpfs/tmpfs.h (num_files, tmpfs_space_used): Use atomic operations
for these variables.
(adjust_used): Use atomic operations.
(get_used): New convenience function to atomically retrieve
tmpfs_space_used.
* tmpfs/node.c (all_nodes_lock): New lock.
(diskfs_alloc_node): Use a separate lock to protect all_nodes.
Adjust the reference counting accordingly.
(diskfs_free_node): Likewise.
(diskfs_cached_lookup):Likewise.
(diskfs_node_iterate): Likewise.
(diskfs_node_norefs): Do not remove the node from all_nodes.  This
actually looks like a mistake, I do not know why they did that here as
well as in diskfs_free_node.
(diskfs_try_dropping_softrefs): Check whether someone reacquired a
reference, and if so hold on to our light reference.
(diskfs_grow): Use atomic operations.
* tmpfs/tmpfs.c (diskfs_set_statfs): Likewise.

1 files changed, 14 insertions, 6 deletions

diff --git a/tmpfs/tmpfs.h b/tmpfs/tmpfs.h
index b3c636d0..ad472009 100644
--- a/tmpfs/tmpfs.h
+++ b/tmpfs/tmpfs.h
@@ -69,17 +69,25 @@ struct tmpfs_dirent
   char name[0];
 };
 
-extern unsigned int num_files;
-extern off_t tmpfs_page_limit, tmpfs_space_used;
-
+extern off_t tmpfs_page_limit;
 extern mach_port_t default_pager;
 
+/* These two must be accessed using atomic operations.  */
+extern unsigned int num_files;
+extern off_t tmpfs_space_used;
+
+/* Convenience function to adjust tmpfs_space_used.  */
 static inline void
 adjust_used (off_t change)
 {
-  pthread_spin_lock (&diskfs_node_refcnt_lock);
-  tmpfs_space_used += change;
-  pthread_spin_unlock (&diskfs_node_refcnt_lock);
+  __atomic_add_fetch (&num_files, change, __ATOMIC_RELAXED);
+}
+
+/* Convenience function to get tmpfs_space_used.  */
+static inline off_t
+get_used (void)
+{
+  return __atomic_load_n (&num_files, __ATOMIC_RELAXED);
 }
 
 #endif