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Subject[PATCH 3.16 59/87] fs/ocfs2: fix race in ocfs2_dentry_attach_lock()
3.16.75-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Wengang Wang <wen.gang.wang@oracle.com>

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@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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
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);
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