gfs2: fix GL_SKIP node_scope problems

Before this patch, when a glock was locked, the very first holder on the
queue would unlock the lockref and call the go_instantiate glops function
(if one existed), unless GL_SKIP was specified. When we introduced the new
node-scope concept, we allowed multiple holders to lock glocks in EX mode
and share the lock.

But node-scope introduced a new problem: if the first holder has GL_SKIP
and the next one does NOT, since it is not the first holder on the queue,
the go_instantiate op was not called. Eventually the GL_SKIP holder may
call the instantiate sub-function (e.g. gfs2_rgrp_bh_get) but there was
still a window of time in which another non-GL_SKIP holder assumes the
instantiate function had been called by the first holder. In the case of
rgrp glocks, this led to a NULL pointer dereference on the buffer_heads.

This patch tries to fix the problem by introducing two new glock flags:

GLF_INSTANTIATE_NEEDED, which keeps track of when the instantiate function
needs to be called to "fill in" or "read in" the object before it is
referenced.

GLF_INSTANTIATE_IN_PROG which is used to determine when a process is
in the process of reading in the object. Whenever a function needs to
reference the object, it checks the GLF_INSTANTIATE_NEEDED flag, and if
set, it sets GLF_INSTANTIATE_IN_PROG and calls the glops "go_instantiate"
function.

As before, the gl_lockref spin_lock is unlocked during the IO operation,
which may take a relatively long amount of time to complete. While
unlocked, if another process determines go_instantiate is still needed,
it sees GLF_INSTANTIATE_IN_PROG is set, and waits for the go_instantiate
glop operation to be completed. Once GLF_INSTANTIATE_IN_PROG is cleared,
it needs to check GLF_INSTANTIATE_NEEDED again because the other process's
go_instantiate operation may not have been successful.

Functions that previously called the instantiate sub-functions now call
directly into gfs2_instantiate so the new bits are managed properly.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/fs/gfs2/glops.c b/fs/gfs2/glops.c
index 8452a83bd55aa1..e2656baf38a38a 100644
--- a/fs/gfs2/glops.c
+++ b/fs/gfs2/glops.c
@@ -357,6 +357,7 @@ static void inode_go_inval(struct gfs2_glock *gl, int flags)
 		truncate_inode_pages(mapping, 0);
 		if (ip) {
 			set_bit(GIF_INVALID, &ip->i_flags);
+			set_bit(GLF_INSTANTIATE_NEEDED, &gl->gl_flags);
 			forget_all_cached_acls(&ip->i_inode);
 			security_inode_invalidate_secctx(&ip->i_inode);
 			gfs2_dir_hash_inval(ip);
@@ -495,13 +496,13 @@ static int inode_go_instantiate(struct gfs2_holder *gh)
 	struct gfs2_inode *ip = gl->gl_object;
 	int error = 0;
 
-	if (!ip)
-		return 0;
+	if (!ip) /* no inode to populate - read it in later */
+		goto out;
 
 	if (test_bit(GIF_INVALID, &ip->i_flags)) {
 		error = gfs2_inode_refresh(ip);
 		if (error)
-			return error;
+			goto out;
 	}
 
 	if (gh->gh_state != LM_ST_DEFERRED)
@@ -515,9 +516,10 @@ static int inode_go_instantiate(struct gfs2_holder *gh)
 			list_add(&ip->i_trunc_list, &sdp->sd_trunc_list);
 		spin_unlock(&sdp->sd_trunc_lock);
 		wake_up(&sdp->sd_quota_wait);
-		return 1;
+		error = 1;
 	}
 
+out:
 	return error;
 }