GFS2: Fix kernel NULL pointer dereference by dlm_astd

This patch fixes a problem in an error path when looking
up dinodes.  There are two sister-functions, gfs2_inode_lookup
and gfs2_process_unlinked_inode.  Both functions acquire and
hold the i_iopen glock for the dinode being looked up. The last
thing they try to do is hold the i_gl glock for the dinode.
If that glock fails for some reason, the error path was
incorrectly calling gfs2_glock_put for the i_iopen glock twice.
This resulted in the glock being prematurely freed.  The
"minimum hold time" usually kept the glock in memory, but the
lock interface to dlm (aka lock_dlm) freed its memory for the
glock.  In some circumstances, it would cause dlm's dlm_astd daemon
to try to call the bast function for the freed lock_dlm memory,
which resulted in a NULL pointer dereference.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/inode.c

@@ -169,7 +169,7 @@ struct inode *gfs2_inode_lookup(struct super_block *sb,
 {
 	struct inode *inode;
 	struct gfs2_inode *ip;
-	struct gfs2_glock *io_gl;
+	struct gfs2_glock *io_gl = NULL;
 	int error;
 
 	inode = gfs2_iget(sb, no_addr);
@@ -198,6 +198,7 @@ struct inode *gfs2_inode_lookup(struct super_block *sb,
 		ip->i_iopen_gh.gh_gl->gl_object = ip;
 
 		gfs2_glock_put(io_gl);
+		io_gl = NULL;
 
 		if ((type == DT_UNKNOWN) && (no_formal_ino == 0))
 			goto gfs2_nfsbypass;
@@ -228,6 +229,7 @@ struct inode *gfs2_inode_lookup(struct super_block *sb,
 fail_glock:
 	gfs2_glock_dq(&ip->i_iopen_gh);
 fail_iopen:
+	if (io_gl)
 		gfs2_glock_put(io_gl);
 fail_put:
 	if (inode->i_state & I_NEW)
@@ -256,7 +258,7 @@ void gfs2_process_unlinked_inode(struct super_block *sb, u64 no_addr)
 {
 	struct gfs2_sbd *sdp;
 	struct gfs2_inode *ip;
-	struct gfs2_glock *io_gl;
+	struct gfs2_glock *io_gl = NULL;
 	int error;
 	struct gfs2_holder gh;
 	struct inode *inode;
@@ -293,6 +295,7 @@ void gfs2_process_unlinked_inode(struct super_block *sb, u64 no_addr)
 
 	ip->i_iopen_gh.gh_gl->gl_object = ip;
 	gfs2_glock_put(io_gl);
+	io_gl = NULL;
 
 	inode->i_mode = DT2IF(DT_UNKNOWN);
 
@@ -319,6 +322,7 @@ void gfs2_process_unlinked_inode(struct super_block *sb, u64 no_addr)
 fail_glock:
 	gfs2_glock_dq(&ip->i_iopen_gh);
 fail_iopen:
+	if (io_gl)
 		gfs2_glock_put(io_gl);
 fail_put:
 	ip->i_gl->gl_object = NULL;