[PATCH 08/15] gfs2: fix an oops in gfs2_permission()

[Al Viro <viro@zeniv.linux.org.uk>]

in RCU mode we might race with gfs2_evict_inode(), which zeroes
->i_gl.  Freeing of the object it points to is RCU-delayed, so
if we manage to fetch the pointer before it's been replaced with
NULL, we are fine.  Check if we'd fetched NULL and treat that
as "bail out and tell the caller to get out of RCU mode".

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/gfs2/inode.c | 6 ++++--
 fs/gfs2/super.c | 2 +-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/gfs2/inode.c b/fs/gfs2/inode.c
index 0eac04507904..e2432c327599 100644
--- a/fs/gfs2/inode.c
+++ b/fs/gfs2/inode.c
@@ -1868,14 +1868,16 @@ int gfs2_permission(struct mnt_idmap *idmap, struct inode *inode,
 {
 	struct gfs2_inode *ip;
 	struct gfs2_holder i_gh;
+	struct gfs2_glock *gl;
 	int error;
 
 	gfs2_holder_mark_uninitialized(&i_gh);
 	ip = GFS2_I(inode);
-	if (gfs2_glock_is_locked_by_me(ip->i_gl) == NULL) {
+	gl = rcu_dereference(ip->i_gl);
+	if (!gl || gfs2_glock_is_locked_by_me(gl) == NULL) {
 		if (mask & MAY_NOT_BLOCK)
 			return -ECHILD;
-		error = gfs2_glock_nq_init(ip->i_gl, LM_ST_SHARED, LM_FLAG_ANY, &i_gh);
+		error = gfs2_glock_nq_init(gl, LM_ST_SHARED, LM_FLAG_ANY, &i_gh);
 		if (error)
 			return error;
 	}
diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
index 02d93da21b2b..0dd5641990b9 100644
--- a/fs/gfs2/super.c
+++ b/fs/gfs2/super.c
@@ -1550,7 +1550,7 @@ static void gfs2_evict_inode(struct inode *inode)
 		wait_on_bit_io(&ip->i_flags, GIF_GLOP_PENDING, TASK_UNINTERRUPTIBLE);
 		gfs2_glock_add_to_lru(ip->i_gl);
 		gfs2_glock_put_eventually(ip->i_gl);
-		ip->i_gl = NULL;
+		rcu_assign_pointer(ip->i_gl, NULL);
 	}
 }
 
-- 
2.39.2