afs: Fix cell removal

Fix cell removal by inserting a more final state than AFS_CELL_FAILED that
indicates that the cell has been unpublished in case the manager is already
requeued and will go through again.  The new AFS_CELL_REMOVED state will
just immediately leave the manager function.

Going through a second time in the AFS_CELL_FAILED state will cause it to
try to remove the cell again, potentially leading to the proc list being
removed.

Fixes: 989782dc ("afs: Overhaul cell database management")
Reported-by: syzbot+b994ecf2b023f14832c1@syzkaller.appspotmail.com
Reported-by: syzbot+0e0db88e1eb44a91ae8d@syzkaller.appspotmail.com
Reported-by: syzbot+2d0585e5efcd43d113c2@syzkaller.appspotmail.com
Reported-by: syzbot+1ecc2f9d3387f1d79d42@syzkaller.appspotmail.com
Reported-by: syzbot+18d51774588492bf3f69@syzkaller.appspotmail.com
Reported-by: syzbot+a5e4946b04d6ca8fa5f3@syzkaller.appspotmail.com
Suggested-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Hillf Danton <hdanton@sina.com>

fs/afs/cell.c

@@ -291,11 +291,11 @@ struct afs_cell *afs_lookup_cell(struct afs_net *net,
 	wait_var_event(&cell->state,
 		       ({
 			       state = smp_load_acquire(&cell->state); /* vs error */
-			       state == AFS_CELL_ACTIVE || state == AFS_CELL_FAILED;
+			       state == AFS_CELL_ACTIVE || state == AFS_CELL_REMOVED;
 		       }));
 
 	/* Check the state obtained from the wait check. */
-	if (state == AFS_CELL_FAILED) {
+	if (state == AFS_CELL_REMOVED) {
 		ret = cell->error;
 		goto error;
 	}
@@ -700,7 +700,6 @@ static void afs_deactivate_cell(struct afs_net *net, struct afs_cell *cell)
 static void afs_manage_cell(struct afs_cell *cell)
 {
 	struct afs_net *net = cell->net;
-	bool deleted;
 	int ret, active;
 
 	_enter("%s", cell->name);
@@ -712,13 +711,15 @@ static void afs_manage_cell(struct afs_cell *cell)
 	case AFS_CELL_FAILED:
 		down_write(&net->cells_lock);
 		active = 1;
-		deleted = atomic_try_cmpxchg_relaxed(&cell->active, &active, 0);
-		if (deleted) {
+		if (atomic_try_cmpxchg_relaxed(&cell->active, &active, 0)) {
 			rb_erase(&cell->net_node, &net->cells);
+			smp_store_release(&cell->state, AFS_CELL_REMOVED);
 		}
 		up_write(&net->cells_lock);
-		if (deleted)
+		if (cell->state == AFS_CELL_REMOVED) {
+			wake_up_var(&cell->state);
 			goto final_destruction;
+		}
 		if (cell->state == AFS_CELL_FAILED)
 			goto done;
 		smp_store_release(&cell->state, AFS_CELL_UNSET);
@@ -760,6 +761,9 @@ static void afs_manage_cell(struct afs_cell *cell)
 		wake_up_var(&cell->state);
 		goto again;
 
+	case AFS_CELL_REMOVED:
+		goto done;
+
 	default:
 		break;
 	}

fs/afs/internal.h

@@ -326,6 +326,7 @@ enum afs_cell_state {
 	AFS_CELL_DEACTIVATING,
 	AFS_CELL_INACTIVE,
 	AFS_CELL_FAILED,
+	AFS_CELL_REMOVED,
 };
 
 /*