[PATCH]nilfs2:missing a read lock for segment writer in nilfs_attach_checkpoint()

[Zhang Qiang <zhangqiang.buaa-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

'ns_cno' of structure 'the_nilfs' must be protected from segment writer, in other words, 
the caller of nilfs_get_check_point should hold read lock for nilfs->ns_segctor_sem. 
This patch adds the lock/unlock operations in nilfs_attach_checkpoint() when calling 
nilfs_cpfile_get_checkpoint().

Signed-off-by: Zhang Qiang <zhangqiang.buaa-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>

---
 fs/nilfs2/super.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/fs/nilfs2/super.c b/fs/nilfs2/super.c
index 05174c5..55f3d6b 100644
--- a/fs/nilfs2/super.c
+++ b/fs/nilfs2/super.c
@@ -371,8 +371,10 @@ int nilfs_attach_checkpoint(struct nilfs_sb_info *sbi, __u64 cno)
 	if (unlikely(err))
 		goto failed;
 
+	down_read(&nilfs->ns_segctor_sem);
 	err = nilfs_cpfile_get_checkpoint(nilfs->ns_cpfile, cno, 0, &raw_cp,
 					  &bh_cp);
+	up_read(&nilfs->ns_segctor_sem);
 	if (unlikely(err)) {
 		if (err == -ENOENT || err == -EINVAL) {
 			printk(KERN_ERR
-- 
1.6.0.6