From: Wengang Wang wen.gang.wang@oracle.com Subject: 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 ---

fs/ocfs2/dcache.c | 12 ++++++++++++ 1 file changed, 12 insertions(+)

--- a/fs/ocfs2/dcache.c~fs-ocfs2-fix-race-in-ocfs2_dentry_attach_lock +++ a/fs/ocfs2/dcache.c @@ -296,6 +296,18 @@ 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); _