fs/ocfs2: fix race in ocfs2_dentry_attach_lock()

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@oracle.com
Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Reported-by: Daniel Sobe <daniel.sobe@nxp.com>
Tested-by: Daniel Sobe <daniel.sobe@nxp.com>
Reviewed-by: Changwei Ge <gechangwei@live.cn>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Gang He <ghe@suse.com>
Cc: Jun Piao <piaojun@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/ocfs2/dcache.c b/fs/ocfs2/dcache.c
index 2d016937fddab856b402c12c686c422a4d7e9ff5..42a61eecdacdd68993a83ceebca5d19939d2d651 100644 (file)
--- a/fs/ocfs2/dcache.c
+++ b/fs/ocfs2/dcache.c
@@ -296,6 +296,18 @@ int ocfs2_dentry_attach_lock(struct dentry *dentry,
 
 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);