GFS2: Introduce helper for clearing gl_object

This patch introduces a new helper function in glock.h that
clears gl_object, with an added integrity check. An additional
integrity check has been added to glock_set_object, plus comments.
This is step 1 in a series to ensure gl_object integrity.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/glock.h

@@ -13,6 +13,7 @@
 #include <linux/sched.h>
 #include <linux/parser.h>
 #include "incore.h"
+#include "util.h"
 
 /* Options for hostdata parser */
 
@@ -257,11 +258,44 @@ static inline bool gfs2_holder_initialized(struct gfs2_holder *gh)
 	return gh->gh_gl;
 }
 
+/**
+ * glock_set_object - set the gl_object field of a glock
+ * @gl: the glock
+ * @object: the object
+ */
 static inline void glock_set_object(struct gfs2_glock *gl, void *object)
 {
 	spin_lock(&gl->gl_lockref.lock);
+	if (gfs2_assert_warn(gl->gl_name.ln_sbd, gl->gl_object == NULL))
+		gfs2_dump_glock(NULL, gl);
 	gl->gl_object = object;
 	spin_unlock(&gl->gl_lockref.lock);
 }
 
+/**
+ * glock_clear_object - clear the gl_object field of a glock
+ * @gl: the glock
+ * @object: the object
+ *
+ * I'd love to similarly add this:
+ *	else if (gfs2_assert_warn(gl->gl_sbd, gl->gl_object == object))
+ *		gfs2_dump_glock(NULL, gl);
+ * Unfortunately, that's not possible because as soon as gfs2_delete_inode
+ * frees the block in the rgrp, another process can reassign it for an I_NEW
+ * inode in gfs2_create_inode because that calls new_inode, not gfs2_iget.
+ * That means gfs2_delete_inode may subsequently try to call this function
+ * for a glock that's already pointing to a brand new inode. If we clear the
+ * new inode's gl_object, we'll introduce metadata corruption. Function
+ * gfs2_delete_inode calls clear_inode which calls gfs2_clear_inode which also
+ * tries to clear gl_object, so it's more than just gfs2_delete_inode.
+ *
+ */
+static inline void glock_clear_object(struct gfs2_glock *gl, void *object)
+{
+	spin_lock(&gl->gl_lockref.lock);
+	if (gl->gl_object == object)
+		gl->gl_object = NULL;
+	spin_unlock(&gl->gl_lockref.lock);
+}
+
 #endif /* __GLOCK_DOT_H__ */

fs/gfs2/inode.c

@@ -201,14 +201,14 @@ struct inode *gfs2_inode_lookup(struct super_block *sb, unsigned int type,
 
 fail_refresh:
 	ip->i_iopen_gh.gh_flags |= GL_NOCACHE;
-	glock_set_object(ip->i_iopen_gh.gh_gl, NULL);
+	glock_clear_object(ip->i_iopen_gh.gh_gl, ip);
 	gfs2_glock_dq_uninit(&ip->i_iopen_gh);
 fail_put:
 	if (io_gl)
 		gfs2_glock_put(io_gl);
 	if (gfs2_holder_initialized(&i_gh))
 		gfs2_glock_dq_uninit(&i_gh);
-	glock_set_object(ip->i_gl, NULL);
+	glock_clear_object(ip->i_gl, ip);
 fail:
 	iget_failed(inode);
 	return ERR_PTR(error);

fs/gfs2/super.c

@@ -1640,13 +1640,13 @@ static void gfs2_evict_inode(struct inode *inode)
 	gfs2_ordered_del_inode(ip);
 	clear_inode(inode);
 	gfs2_dir_hash_inval(ip);
-	glock_set_object(ip->i_gl, NULL);
+	glock_clear_object(ip->i_gl, ip);
 	wait_on_bit_io(&ip->i_flags, GIF_GLOP_PENDING, TASK_UNINTERRUPTIBLE);
 	gfs2_glock_add_to_lru(ip->i_gl);
 	gfs2_glock_put(ip->i_gl);
 	ip->i_gl = NULL;
 	if (gfs2_holder_initialized(&ip->i_iopen_gh)) {
-		glock_set_object(ip->i_iopen_gh.gh_gl, NULL);
+		glock_clear_object(ip->i_iopen_gh.gh_gl, ip);
 		ip->i_iopen_gh.gh_flags |= GL_NOCACHE;
 		gfs2_glock_dq_uninit(&ip->i_iopen_gh);
 	}