no_cpu_relax hack

Getting stashed dentry only for the bpf helper does not seem to work -
the dentry is usually zero, so the fast path returns ENOENT. Going
with slow path for the bpf helper is kind of icky, because the slow
path can sleep.

Since we don't want to sleep during the bpf helper function, allow
going through the slow path without calling cpu_relax. All other
callers of the function with the slow path are not affected.

It kinda still stinks, because the bpf helper still has some busy
loop…

Signed-off-by: Krzesimir Nowak <[email protected]>

Author: krnowak

fs/nsfs.c

@@ -70,7 +70,13 @@ static struct dentry *__ns_get_stashed_dentry(struct ns_common *ns)
 	return dentry;
 }
 
-static void *__ns_get_path(struct path *path, struct ns_common *ns)
+enum ns_get_path_cpu_relax_mode {
+	WITH_CPU_RELAX,
+	NO_CPU_RELAX
+};
+
+static void *__ns_get_path(struct path *path, struct ns_common *ns,
+			   enum ns_get_path_cpu_relax_mode cpu_relax_mode)
 {
 	struct vfsmount *mnt = nsfs_mnt;
 	struct dentry *dentry;
@@ -106,7 +112,8 @@ static void *__ns_get_path(struct path *path, struct ns_common *ns)
 	if (d) {
 		d_delete(dentry);	/* make sure ->d_prune() does nothing */
 		dput(dentry);
-		cpu_relax();
+		if (cpu_relax_mode == WITH_CPU_RELAX)
+			cpu_relax();
 		return ERR_PTR(-EAGAIN);
 	}
 got_it:
@@ -115,9 +122,28 @@ static void *__ns_get_path(struct path *path, struct ns_common *ns)
 	return NULL;
 }
 
+static void *__ns_get_path_cb(struct path *path, ns_get_path_helper_t *ns_get_cb,
+			      void *private_data,
+			      enum ns_get_path_cpu_relax_mode cpu_relax_mode)
+{
+	void *ret;
+
+	do {
+		struct ns_common *ns = ns_get_cb(private_data);
+		if (!ns)
+			return ERR_PTR(-ENOENT);
+
+		ret = __ns_get_path(path, ns, cpu_relax_mode);
+	} while (ret == ERR_PTR(-EAGAIN));
+
+	return ret;
+}
+
 void *ns_get_path_cb_fast(struct path *path, ns_get_path_helper_t *ns_get_cb,
 			  void *private_data)
 {
+	return __ns_get_path_cb(path, ns_get_cb, private_data, NO_CPU_RELAX);
+	/*
 	struct ns_common *ns = ns_get_cb(private_data);
 	struct dentry *dentry;
 
@@ -130,22 +156,13 @@ void *ns_get_path_cb_fast(struct path *path, ns_get_path_helper_t *ns_get_cb,
 	path->mnt = mntget(nsfs_mnt);
 	path->dentry = dentry;
 	return NULL;
+	*/
 }
 
 void *ns_get_path_cb(struct path *path, ns_get_path_helper_t *ns_get_cb,
 		     void *private_data)
 {
-	void *ret;
-
-	do {
-		struct ns_common *ns = ns_get_cb(private_data);
-		if (!ns)
-			return ERR_PTR(-ENOENT);
-
-		ret = __ns_get_path(path, ns);
-	} while (ret == ERR_PTR(-EAGAIN));
-
-	return ret;
+	return __ns_get_path_cb(path, ns_get_cb, private_data, WITH_CPU_RELAX);
 }
 
 struct ns_get_path_task_args {
@@ -192,7 +209,7 @@ int open_related_ns(struct ns_common *ns,
 			return PTR_ERR(relative);
 		}
 
-		err = __ns_get_path(&path, relative);
+		err = __ns_get_path(&path, relative, WITH_CPU_RELAX);
 	} while (err == ERR_PTR(-EAGAIN));
 
 	if (IS_ERR(err)) {