1 files changed, 61 insertions, 9 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index b70331f9f5b7..365586a4c4de 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1261,16 +1261,68 @@ struct super_operations {
 #endif
 };
 
-/* Inode state bits.  Protected by inode_lock. */
-#define I_DIRTY_SYNC            1 /* Not dirty enough for O_DATASYNC */
-#define I_DIRTY_DATASYNC        2 /* Data-related inode changes pending */
-#define I_DIRTY_PAGES           4 /* Data-related inode changes pending */
-#define __I_LOCK                3
+/*
+ * Inode state bits.  Protected by inode_lock.
+ *
+ * Three bits determine the dirty state of the inode, I_DIRTY_SYNC,
+ * I_DIRTY_DATASYNC and I_DIRTY_PAGES.
+ *
+ * Four bits define the lifetime of an inode.  Initially, inodes are I_NEW,
+ * until that flag is cleared.  I_WILL_FREE, I_FREEING and I_CLEAR are set at
+ * various stages of removing an inode.
+ *
+ * Two bits are used for locking and completion notification, I_LOCK and I_SYNC.
+ *
+ * I_DIRTY_SYNC         Inode itself is dirty.
+ * I_DIRTY_DATASYNC     Data-related inode changes pending
+ * I_DIRTY_PAGES        Inode has dirty pages.  Inode itself may be clean.
+ * I_NEW                get_new_inode() sets i_state to I_LOCK|I_NEW.  Both
+ *                      are cleared by unlock_new_inode(), called from iget().
+ * I_WILL_FREE          Must be set when calling write_inode_now() if i_count
+ *                      is zero.  I_FREEING must be set when I_WILL_FREE is
+ *                      cleared.
+ * I_FREEING            Set when inode is about to be freed but still has dirty
+ *                      pages or buffers attached or the inode itself is still
+ *                      dirty.
+ * I_CLEAR              Set by clear_inode().  In this state the inode is clean
+ *                      and can be destroyed.
+ *
+ *                      Inodes that are I_WILL_FREE, I_FREEING or I_CLEAR are
+ *                      prohibited for many purposes.  iget() must wait for
+ *                      the inode to be completely released, then create it
+ *                      anew.  Other functions will just ignore such inodes,
+ *                      if appropriate.  I_LOCK is used for waiting.
+ *
+ * I_LOCK               Serves as both a mutex and completion notification.
+ *                      New inodes set I_LOCK.  If two processes both create
+ *                      the same inode, one of them will release its inode and
+ *                      wait for I_LOCK to be released before returning.
+ *                      Inodes in I_WILL_FREE, I_FREEING or I_CLEAR state can
+ *                      also cause waiting on I_LOCK, without I_LOCK actually
+ *                      being set.  find_inode() uses this to prevent returning
+ *                      nearly-dead inodes.
+ * I_SYNC               Similar to I_LOCK, but limited in scope to writeback
+ *                      of inode dirty data.  Having a seperate lock for this
+ *                      purpose reduces latency and prevents some filesystem-
+ *                      specific deadlocks.
+ *
+ * Q: Why does I_DIRTY_DATASYNC exist?  It appears as if it could be replaced
+ *    by (I_DIRTY_SYNC|I_DIRTY_PAGES).
+ * Q: What is the difference between I_WILL_FREE and I_FREEING?
+ * Q: igrab() only checks on (I_FREEING|I_WILL_FREE).  Should it also check on
+ *    I_CLEAR?  If not, why?
+ */
+#define I_DIRTY_SYNC            1
+#define I_DIRTY_DATASYNC        2
+#define I_DIRTY_PAGES           4
+#define I_NEW                   8
+#define I_WILL_FREE             16
+#define I_FREEING               32
+#define I_CLEAR                 64
+#define __I_LOCK                7
 #define I_LOCK                  (1 << __I_LOCK)
-#define I_FREEING               16
-#define I_CLEAR                 32
-#define I_NEW                   64
-#define I_WILL_FREE             128
+#define __I_SYNC                8
+#define I_SYNC                  (1 << __I_SYNC)
 
 #define I_DIRTY (I_DIRTY_SYNC | I_DIRTY_DATASYNC | I_DIRTY_PAGES)