[PATCH v4 2/2] ocfs2: snapshot the dentry name while tracing file release

[Cen Zhang <zzzccc427@gmail.com>]

ocfs2_file_release() passes the live dentry name and length to a trace
event while holding oi->ip_lock. This lock protects the open count and
delays RCU reclamation of external names, but it does not serialize name
changes during rename. The dentry can switch between inline and external
names, and a separately loaded length can describe a different name.

The ocfs2__file_ops event sizes and copies its __string separately.
A concurrently rewritten inline name need not remain consistent between
those operations. External names can also be selected at this call site;
the existing spinlock critical section protects their allocation lifetime,
while a snapshot additionally pairs a stable name with its length.

Take a name snapshot before acquiring oi->ip_lock and release it after
dropping the lock. Keep the trace call and open-count argument inside
the original critical section. Cache the event-enabled check so the
snapshot stays paired even if tracing is toggled meanwhile.

Fixes: 468eedde23d6 ("ocfs2: Remove mlog(0) from fs/ocfs2/file.c")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---
 fs/ocfs2/file.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
index c6e7606ff9e1..b3a37b9d259a 100644
--- a/fs/ocfs2/file.c
+++ b/fs/ocfs2/file.c
@@ -141,18 +141,26 @@ leave:
 static int ocfs2_file_release(struct inode *inode, struct file *file)
 {
 	struct ocfs2_inode_info *oi = OCFS2_I(inode);
+	struct dentry *dentry = file->f_path.dentry;
+	struct name_snapshot name;
+	bool trace_enabled = trace_ocfs2_file_release_enabled();
+
+	if (trace_enabled)
+		take_dentry_name_snapshot(&name, dentry);
 
 	spin_lock(&oi->ip_lock);
 	if (!--oi->ip_open_count)
 		oi->ip_flags &= ~OCFS2_INODE_OPEN_DIRECT;
 
-	trace_ocfs2_file_release(inode, file, file->f_path.dentry,
-				 oi->ip_blkno,
-				 file->f_path.dentry->d_name.len,
-				 file->f_path.dentry->d_name.name,
-				 oi->ip_open_count);
+	if (trace_enabled)
+		trace_ocfs2_file_release(inode, file, dentry,
+					 oi->ip_blkno, name.name.len,
+					 name.name.name, oi->ip_open_count);
 	spin_unlock(&oi->ip_lock);
 
+	if (trace_enabled)
+		release_dentry_name_snapshot(&name);
+
 	ocfs2_free_file_private(inode, file);
 
 	return 0;