[PATCH v3 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. That lock protects the open
count, but it does not serialize dentry name changes during rename.
The ocfs2__file_ops event sizes and copies its __string separately,
so a concurrently rewritten inline name need not remain consistent
between those operations. The separately loaded length may also
describe a different name.

  Release trace                        Renamer
  -------------                        -------
  hold oi->ip_lock
  size the inline name
                                       rewrite the inline name
                                       under dentry->d_lock
  copy the changed name

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 is paired even if tracing is toggled meanwhile.
The snapshot keeps the trace name stable without extending the
inode lock over snapshot acquisition.

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 3c4637b32c09..8da73d0707ce 100644
--- a/fs/ocfs2/file.c
+++ b/fs/ocfs2/file.c
@@ -141,18 +141,26 @@ static int ocfs2_file_open(struct inode *inode, struct file *file)
 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;