[PATCH 6.18.y 2/6] fs: add missing fences to I_NEW handling

[Sasha Levin <sashal@kernel.org>]

From: Mateusz Guzik <mjguzik@gmail.com>

[ Upstream commit 31e332b911fca54df467d264d7e2a2ef9317f3ca ]

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>
Stable-dep-of: 8ee09f597763 ("fs: rotate inodes whose page cache could not be dropped")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/dcache.c               | 4 ++++
 fs/inode.c                | 8 ++++++++
 include/linux/writeback.h | 4 ++++
 3 files changed, 16 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index 3ad74a3abf9ad..84952f6bbf406 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1995,6 +1995,10 @@ 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
diff --git a/fs/inode.c b/fs/inode.c
index acf09822535ed..7adfc787e934a 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1177,6 +1177,10 @@ 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
@@ -1194,6 +1198,10 @@ 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
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 22dd4adc5667d..e1e1231a68303 100644
--- a/include/linux/writeback.h
+++ b/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
-- 
2.53.0