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>

3 files changed, 31 insertions, 29 deletions

diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
index 591202bc9668..1f72b00447c4 100644
--- a/fs/ceph/caps.c
+++ b/fs/ceph/caps.c
@@ -2635,6 +2635,7 @@ static void handle_cap_export(struct inode *inode, struct ceph_mds_caps *ex,
                               struct ceph_mds_session *session,
                               int *open_target_sessions)
 {
+        struct ceph_mds_client *mdsc = ceph_inode_to_client(inode)->mdsc;
         struct ceph_inode_info *ci = ceph_inode(inode);
         int mds = session->s_mds;
         unsigned mseq = le32_to_cpu(ex->migrate_seq);
@@ -2671,6 +2672,19 @@ static void handle_cap_export(struct inode *inode, struct ceph_mds_caps *ex,
                          * export targets, so that we get the matching IMPORT
                          */
                         *open_target_sessions = 1;
+
+                        /*
+                         * we can't flush dirty caps that we've seen the
+                         * EXPORT but no IMPORT for
+                         */
+                        spin_lock(&mdsc->cap_dirty_lock);
+                        if (!list_empty(&ci->i_dirty_item)) {
+                                dout(" moving %p to cap_dirty_migrating\n",
+                                     inode);
+                                list_move(&ci->i_dirty_item,
+                                          &mdsc->cap_dirty_migrating);
+                        }
+                        spin_unlock(&mdsc->cap_dirty_lock);
                 }
                 __ceph_remove_cap(cap);
         }
@@ -2708,6 +2722,13 @@ static void handle_cap_import(struct ceph_mds_client *mdsc,
                 ci->i_cap_exporting_issued = 0;
                 ci->i_cap_exporting_mseq = 0;
                 ci->i_cap_exporting_mds = -1;
+
+                spin_lock(&mdsc->cap_dirty_lock);
+                if (!list_empty(&ci->i_dirty_item)) {
+                        dout(" moving %p back to cap_dirty\n", inode);
+                        list_move(&ci->i_dirty_item, &mdsc->cap_dirty);
+                }
+                spin_unlock(&mdsc->cap_dirty_lock);
         } else {
                 dout("handle_cap_import inode %p ci %p mds%d mseq %d\n",
                      inode, ci, mds, mseq);
@@ -2911,38 +2932,16 @@ void ceph_check_delayed_caps(struct ceph_mds_client *mdsc)
  */
 void ceph_flush_dirty_caps(struct ceph_mds_client *mdsc)
 {
-        struct ceph_inode_info *ci, *nci = NULL;
-        struct inode *inode, *ninode = NULL;
-        struct list_head *p, *n;
+        struct ceph_inode_info *ci;
+        struct inode *inode;
 
         dout("flush_dirty_caps\n");
         spin_lock(&mdsc->cap_dirty_lock);
-        list_for_each_safe(p, n, &mdsc->cap_dirty) {
-                if (nci) {
-                        ci = nci;
-                        inode = ninode;
-                        ci->i_ceph_flags &= ~CEPH_I_NOFLUSH;
-                        dout("flush_dirty_caps inode %p (was next inode)\n",
-                             inode);
-                } else {
-                        ci = list_entry(p, struct ceph_inode_info,
-                                        i_dirty_item);
-                        inode = igrab(&ci->vfs_inode);
-                        BUG_ON(!inode);
-                        dout("flush_dirty_caps inode %p\n", inode);
-                }
-                if (n != &mdsc->cap_dirty) {
-                        nci = list_entry(n, struct ceph_inode_info,
-                                         i_dirty_item);
-                        ninode = igrab(&nci->vfs_inode);
-                        BUG_ON(!ninode);
-                        nci->i_ceph_flags |= CEPH_I_NOFLUSH;
-                        dout("flush_dirty_caps next inode %p, noflush\n",
-                             ninode);
-                } else {
-                        nci = NULL;
-                        ninode = NULL;
-                }
+        while (!list_empty(&mdsc->cap_dirty)) {
+                ci = list_first_entry(&mdsc->cap_dirty, struct ceph_inode_info,
+                                      i_dirty_item);
+                inode = igrab(&ci->vfs_inode);
+                dout("flush_dirty_caps %p\n", inode);
                 spin_unlock(&mdsc->cap_dirty_lock);
                 if (inode) {
                         ceph_check_caps(ci, CHECK_CAPS_NODELAY|CHECK_CAPS_FLUSH,
@@ -2952,6 +2951,7 @@ void ceph_flush_dirty_caps(struct ceph_mds_client *mdsc)
                 spin_lock(&mdsc->cap_dirty_lock);
         }
         spin_unlock(&mdsc->cap_dirty_lock);
+        dout("flush_dirty_caps done\n");
 }
 
 /*
diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index c12d2e9a0ec6..79743d146be6 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);
diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
index 4e3a9cc0bba6..7d8a0d662d56 100644
--- a/fs/ceph/mds_client.h
+++ b/fs/ceph/mds_client.h
@@ -278,6 +278,7 @@ struct ceph_mds_client {
 
         u64               cap_flush_seq;
         struct list_head  cap_dirty;        /* inodes with dirty caps */
+        struct list_head  cap_dirty_migrating; /* ...that are migration... */
         int               num_cap_flushing; /* # caps we are flushing */
         spinlock_t        cap_dirty_lock;   /* protects above items */
         wait_queue_head_t cap_flushing_wq;