ceph: fix cap flush race reentrancy

In e9964c10 we change cap flushing to do a delicate dance because some
inodes on the cap_dirty list could be in a migrating state (got EXPORT but
not IMPORT) in which we couldn't actually flush and move from
dirty->flushing, breaking the while (!empty) { process first } loop
structure.  It worked for a single sync thread, but was not reentrant and
triggered infinite loops when multiple syncers came along.

Instead, move inodes with dirty to a separate cap_dirty_migrating list
when in the limbo export-but-no-import state, allowing us to go back to
the simple loop structure (which was reentrant).  This is cleaner and more
robust.

Audited the cap_dirty users and this looks fine:
list_empty(&ci->i_dirty_item) is still a reliable indicator of whether we
have dirty caps (which list we're on is irrelevant) and list_del_init()
calls still do the right thing.

Signed-off-by: Sage Weil <sage@newdream.net>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index c12d2e9..79743d1 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -3004,6 +3004,7 @@ int ceph_mdsc_init(struct ceph_fs_client *fsc)
 	spin_lock_init(&mdsc->snap_flush_lock);
 	mdsc->cap_flush_seq = 0;
 	INIT_LIST_HEAD(&mdsc->cap_dirty);
+	INIT_LIST_HEAD(&mdsc->cap_dirty_migrating);
 	mdsc->num_cap_flushing = 0;
 	spin_lock_init(&mdsc->cap_dirty_lock);
 	init_waitqueue_head(&mdsc->cap_flushing_wq);