fs: provide accessors for ->i_state

Open-coded accesses prevent asserting they are done correctly. One
obvious aspect is locking, but significantly more can checked. For
example it can be detected when the code is clearing flags which are
already missing, or is setting flags when it is illegal (e.g., I_FREEING
when ->i_count > 0).

In order to keep things manageable this patchset merely gets the thing
off the ground with only lockdep checks baked in.

Current consumers can be trivially converted.

Suppose flags I_A and I_B are to be handled.

If ->i_lock is held, then:

state = inode->i_state  	=> state = inode_state_read(inode)
inode->i_state |= (I_A | I_B) 	=> inode_state_set(inode, I_A | I_B)
inode->i_state &= ~(I_A | I_B) 	=> inode_state_clear(inode, I_A | I_B)
inode->i_state = I_A | I_B	=> inode_state_assign(inode, I_A | I_B)

If ->i_lock is not held or only held conditionally:

state = inode->i_state  	=> state = inode_state_read_once(inode)
inode->i_state |= (I_A | I_B) 	=> inode_state_set_raw(inode, I_A | I_B)
inode->i_state &= ~(I_A | I_B) 	=> inode_state_clear_raw(inode, I_A | I_B)
inode->i_state = I_A | I_B	=> inode_state_assign_raw(inode, I_A | I_B)

The "_once" vs "_raw" discrepancy stems from the read variant differing
by READ_ONCE as opposed to just lockdep checks.

Finally, if you want to atomically clear flags and set new ones, the
following:

state = inode->i_state;
state &= ~I_A;
state |= I_B;
inode->i_state = state;

turns into:

inode_state_replace(inode, I_A, I_B);

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/fs.h

@@ -759,7 +759,7 @@ enum inode_state_bits {
 	/* reserved wait address bit 3 */
 };
 
-enum inode_state_flags_t {
+enum inode_state_flags_enum {
 	I_NEW			= (1U << __I_NEW),
 	I_SYNC			= (1U << __I_SYNC),
 	I_LRU_ISOLATING         = (1U << __I_LRU_ISOLATING),
@@ -843,7 +843,7 @@ struct inode {
 #endif
 
 	/* Misc */
-	enum inode_state_flags_t	i_state;
+	enum inode_state_flags_enum i_state;
 	/* 32-bit hole */
 	struct rw_semaphore	i_rwsem;
 
@@ -902,6 +902,80 @@ struct inode {
 	void			*i_private; /* fs or device private pointer */
 } __randomize_layout;
 
+/*
+ * i_state handling
+ *
+ * We hide all of it behind helpers so that we can validate consumers.
+ */
+static inline enum inode_state_flags_enum inode_state_read_once(struct inode *inode)
+{
+	return READ_ONCE(inode->i_state);
+}
+
+static inline enum inode_state_flags_enum inode_state_read(struct inode *inode)
+{
+	lockdep_assert_held(&inode->i_lock);
+	return inode->i_state;
+}
+
+static inline void inode_state_set_raw(struct inode *inode,
+				       enum inode_state_flags_enum flags)
+{
+	WRITE_ONCE(inode->i_state, inode->i_state | flags);
+}
+
+static inline void inode_state_set(struct inode *inode,
+				   enum inode_state_flags_enum flags)
+{
+	lockdep_assert_held(&inode->i_lock);
+	inode_state_set_raw(inode, flags);
+}
+
+static inline void inode_state_clear_raw(struct inode *inode,
+					 enum inode_state_flags_enum flags)
+{
+	WRITE_ONCE(inode->i_state, inode->i_state & ~flags);
+}
+
+static inline void inode_state_clear(struct inode *inode,
+				     enum inode_state_flags_enum flags)
+{
+	lockdep_assert_held(&inode->i_lock);
+	inode_state_clear_raw(inode, flags);
+}
+
+static inline void inode_state_assign_raw(struct inode *inode,
+					  enum inode_state_flags_enum flags)
+{
+	WRITE_ONCE(inode->i_state, flags);
+}
+
+static inline void inode_state_assign(struct inode *inode,
+				      enum inode_state_flags_enum flags)
+{
+	lockdep_assert_held(&inode->i_lock);
+	inode_state_assign_raw(inode, flags);
+}
+
+static inline void inode_state_replace_raw(struct inode *inode,
+					   enum inode_state_flags_enum clearflags,
+					   enum inode_state_flags_enum setflags)
+{
+	enum inode_state_flags_enum flags;
+	flags = inode->i_state;
+	flags &= ~clearflags;
+	flags |= setflags;
+	inode_state_assign_raw(inode, flags);
+}
+
+static inline void inode_state_replace(struct inode *inode,
+				       enum inode_state_flags_enum clearflags,
+				       enum inode_state_flags_enum setflags)
+{
+	lockdep_assert_held(&inode->i_lock);
+	inode_state_replace_raw(inode, clearflags, setflags);
+}
+
 static inline void inode_set_cached_link(struct inode *inode, char *link, int linklen)
 {
 	VFS_WARN_ON_INODE(strlen(link) != linklen, inode);