gfs2: fix quota refresh race in do_glock()

quotad periodically syncs in-memory quotas to the ondisk quota file
and sets the QDF_REFRESH flag so that a subsequent read of a synced
quota is re-read from disk.

gfs2_quota_lock() checks for this flag and sets a 'force' bit to
force re-read from disk if requested. However, there is a race
condition here. It is possible for gfs2_quota_lock() to find the
QDF_REFRESH flag unset (i.e force=0) and quotad comes in immediately
after and syncs the relevant quota and sets the QDF_REFRESH flag.
gfs2_quota_lock() resumes with force=0 and uses the stale in-memory
quota usage values that result in miscalculations.

This patch fixes this race by moving the check for the QDF_REFRESH
flag check further out into the gfs2_quota_lock() process, i.e, in
do_glock(), under the protection of the quota glock.

Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Acked-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/quota.c

@@ -923,6 +923,9 @@ static int do_glock(struct gfs2_quota_data *qd, int force_refresh,
 	if (error)
 		return error;
 
+	if (test_and_clear_bit(QDF_REFRESH, &qd->qd_flags))
+		force_refresh = FORCE;
+
 	qd->qd_qb = *(struct gfs2_quota_lvb *)qd->qd_gl->gl_lksb.sb_lvbptr;
 
 	if (force_refresh || qd->qd_qb.qb_magic != cpu_to_be32(GFS2_MAGIC)) {
@@ -974,11 +977,8 @@ int gfs2_quota_lock(struct gfs2_inode *ip, kuid_t uid, kgid_t gid)
 	     sizeof(struct gfs2_quota_data *), sort_qd, NULL);
 
 	for (x = 0; x < ip->i_res->rs_qa_qd_num; x++) {
-		int force = NO_FORCE;
 		qd = ip->i_res->rs_qa_qd[x];
-		if (test_and_clear_bit(QDF_REFRESH, &qd->qd_flags))
-			force = FORCE;
-		error = do_glock(qd, force, &ip->i_res->rs_qa_qd_ghs[x]);
+		error = do_glock(qd, NO_FORCE, &ip->i_res->rs_qa_qd_ghs[x]);
 		if (error)
 			break;
 	}