fs: add missing fences to I_NEW handling

Suppose there are 2 CPUs racing inode hash lookup func (say ilookup5())
and unlock_new_inode().

In principle the latter can clear the I_NEW flag before prior stores
into the inode were made visible.

The former can in turn observe I_NEW is cleared and proceed to use the
inode, while possibly reading from not-yet-published areas.

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

fs/dcache.c

@@ -1981,6 +1981,10 @@ void d_instantiate_new(struct dentry *entry, struct inode *inode)
 	spin_lock(&inode->i_lock);
 	__d_instantiate(entry, inode);
 	WARN_ON(!(inode->i_state & I_NEW));
+	/*
+	 * Pairs with smp_rmb in wait_on_inode().
+	 */
+	smp_wmb();
 	inode->i_state &= ~I_NEW & ~I_CREATING;
 	/*
 	 * Pairs with the barrier in prepare_to_wait_event() to make sure

fs/inode.c

@@ -1181,6 +1181,10 @@ void unlock_new_inode(struct inode *inode)
 	lockdep_annotate_inode_mutex_key(inode);
 	spin_lock(&inode->i_lock);
 	WARN_ON(!(inode->i_state & I_NEW));
+	/*
+	 * Pairs with smp_rmb in wait_on_inode().
+	 */
+	smp_wmb();
 	inode->i_state &= ~I_NEW & ~I_CREATING;
 	/*
 	 * Pairs with the barrier in prepare_to_wait_event() to make sure
@@ -1198,6 +1202,10 @@ void discard_new_inode(struct inode *inode)
 	lockdep_annotate_inode_mutex_key(inode);
 	spin_lock(&inode->i_lock);
 	WARN_ON(!(inode->i_state & I_NEW));
+	/*
+	 * Pairs with smp_rmb in wait_on_inode().
+	 */
+	smp_wmb();
 	inode->i_state &= ~I_NEW;
 	/*
 	 * Pairs with the barrier in prepare_to_wait_event() to make sure

include/linux/writeback.h

@@ -194,6 +194,10 @@ static inline void wait_on_inode(struct inode *inode)
 {
 	wait_var_event(inode_state_wait_address(inode, __I_NEW),
 		       !(READ_ONCE(inode->i_state) & I_NEW));
+	/*
+	 * Pairs with routines clearing I_NEW.
+	 */
+	smp_rmb();
 }
 
 #ifdef CONFIG_CGROUP_WRITEBACK