NFSD: WARN when freeing an item still linked via nf_lru

Add a guardrail to prevent freeing memory that is still on a list.
This includes either a dispose list or the LRU list.

This is the sign of a bug, but this class of bugs can be detected
so that they don't endanger system stability, especially while
debugging.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/filecache.c

@@ -214,6 +214,14 @@ nfsd_file_free(struct nfsd_file *nf)
 		fput(nf->nf_file);
 		flush = true;
 	}
+
+	/*
+	 * If this item is still linked via nf_lru, that's a bug.
+	 * WARN and leak it to preserve system stability.
+	 */
+	if (WARN_ON_ONCE(!list_empty(&nf->nf_lru)))
+		return flush;
+
 	call_rcu(&nf->nf_rcu, nfsd_file_slab_free);
 	return flush;
 }
@@ -343,7 +351,7 @@ nfsd_file_dispose_list(struct list_head *dispose)
 
 	while(!list_empty(dispose)) {
 		nf = list_first_entry(dispose, struct nfsd_file, nf_lru);
-		list_del(&nf->nf_lru);
+		list_del_init(&nf->nf_lru);
 		nfsd_file_flush(nf);
 		nfsd_file_put_noref(nf);
 	}
@@ -357,7 +365,7 @@ nfsd_file_dispose_list_sync(struct list_head *dispose)
 
 	while(!list_empty(dispose)) {
 		nf = list_first_entry(dispose, struct nfsd_file, nf_lru);
-		list_del(&nf->nf_lru);
+		list_del_init(&nf->nf_lru);
 		nfsd_file_flush(nf);
 		if (!refcount_dec_and_test(&nf->nf_ref))
 			continue;