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>

diff --git a/fs/gfs2/quota.c b/fs/gfs2/quota.c
index 5561468..5c27e48 100644 (file)
--- a/fs/gfs2/quota.c
+++ b/fs/gfs2/quota.c
@@ -923,6 +923,9 @@ restart:
        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;
        }