[PATCH 3.16 59/87] fs/ocfs2: fix race in ocfs2_dentry_attach_lock()

[Ben Hutchings]

3.16.75-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Wengang Wang <wen.gang.wang@xxxxxxxxxx>

commit be99ca2716972a712cde46092c54dee5e6192bf8 upstream.

ocfs2_dentry_attach_lock() can be executed in parallel threads against the
same dentry.  Make that race safe.  The race is like this:

            thread A                               thread B

(A1) enter ocfs2_dentry_attach_lock,
seeing dentry->d_fsdata is NULL,
and no alias found by
ocfs2_find_local_alias, so kmalloc
a new ocfs2_dentry_lock structure
to local variable "dl", dl1

               .....

                                    (B1) enter ocfs2_dentry_attach_lock,
                                    seeing dentry->d_fsdata is NULL,
                                    and no alias found by
                                    ocfs2_find_local_alias so kmalloc
                                    a new ocfs2_dentry_lock structure
                                    to local variable "dl", dl2.

                                                   ......

(A2) set dentry->d_fsdata with dl1,
call ocfs2_dentry_lock() and increase
dl1->dl_lockres.l_ro_holders to 1 on
success.
              ......

                                    (B2) set dentry->d_fsdata with dl2
                                    call ocfs2_dentry_lock() and increase
				    dl2->dl_lockres.l_ro_holders to 1 on
				    success.

                                                  ......

(A3) call ocfs2_dentry_unlock()
and decrease
dl2->dl_lockres.l_ro_holders to 0
on success.
             ....

                                    (B3) call ocfs2_dentry_unlock(),
                                    decreasing
				    dl2->dl_lockres.l_ro_holders, but
				    see it's zero now, panic

Link: http://lkml.kernel.org/r/20190529174636.22364-1-wen.gang.wang@xxxxxxxxxx
Signed-off-by: Wengang Wang <wen.gang.wang@xxxxxxxxxx>
Reported-by: Daniel Sobe <daniel.sobe@xxxxxxx>
Tested-by: Daniel Sobe <daniel.sobe@xxxxxxx>
Reviewed-by: Changwei Ge <gechangwei@xxxxxxx>
Reviewed-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
Cc: Mark Fasheh <mark@xxxxxxxxxx>
Cc: Joel Becker <jlbec@xxxxxxxxxxxx>
Cc: Junxiao Bi <junxiao.bi@xxxxxxxxxx>
Cc: Gang He <ghe@xxxxxxxx>
Cc: Jun Piao <piaojun@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/ocfs2/dcache.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

--- a/fs/ocfs2/dcache.c
+++ b/fs/ocfs2/dcache.c
@@ -310,6 +310,18 @@ int ocfs2_dentry_attach_lock(struct dent
 
 out_attach:
 	spin_lock(&dentry_attach_lock);
+	if (unlikely(dentry->d_fsdata && !alias)) {
+		/* d_fsdata is set by a racing thread which is doing
+		 * the same thing as this thread is doing. Leave the racing
+		 * thread going ahead and we return here.
+		 */
+		spin_unlock(&dentry_attach_lock);
+		iput(dl->dl_inode);
+		ocfs2_lock_res_free(&dl->dl_lockres);
+		kfree(dl);
+		return 0;
+	}
+
 	dentry->d_fsdata = dl;
 	dl->dl_count++;
 	spin_unlock(&dentry_attach_lock);