linux-kernel - [PATCH] fs: add missing fences to I

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Message-ID: <20251005231526.708061-1-mjguzik@gmail.com>
Date: Mon,  6 Oct 2025 01:15:26 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
	jack@...e.cz,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] 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@...il.com>
---

I don't think this is a serious bug in the sense I doubt anyone ever ran
into it, but this is an issue on paper.

I'm doing some changes in the area and I figured I'll get this bit out
of the way.

 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 a067fa0a965a..806d6a665124 100644
--- a/fs/dcache.c
+++ b/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
diff --git a/fs/inode.c b/fs/inode.c
index ec9339024ac3..842ee973c8b6 100644
--- a/fs/inode.c
+++ b/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
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 22dd4adc5667..e1e1231a6830 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
-- 
2.34.1