[v2] ovl: lockdep annotate of nested stacked overlayfs inode lock
diff mbox

Message ID 1480784286-27229-1-git-send-email-amir73il@gmail.com
State New
Headers show

Commit Message

Amir Goldstein Dec. 3, 2016, 4:58 p.m. UTC
An overlayfs instance can be the lower layer of another overlayfs
instance. This setup triggers a lockdep splat of possible recursive
locking of sb->s_type->i_mutex_key in iterate_dir(). Trimmed snip:

 [ INFO: possible recursive locking detected ]
 bash/2468 is trying to acquire lock:
  &sb->s_type->i_mutex_key#14, at: iterate_dir+0x7d/0x15c
 but task is already holding lock:
  &sb->s_type->i_mutex_key#14, at: iterate_dir+0x7d/0x15c

One problem observed with this splat is that ovl_new_inode()
does not call lockdep_annotate_inode_mutex_key() to annotate
the dir inode lock as &sb->s_type->i_mutex_dir_key like other
fs do.

The other problem is that the 2 nested levels of overlayfs inode
lock are annotated using the same key, which is the cause of the
false positive lockdep warning.

Fix this by annotating overlayfs inode lock in ovl_fill_inode()
according to stack level of the super block instance and use
different key for dir vs. non-dir.

Also, annotate the first level of overlayfs as 'ovl_i_mutex_key'
and higher levels of nesting as '&ovl_i_mutex_dir_key[nested]'
to make it easier to read lockdep reports.
Here is an edited snip from /proc/lockdep_chains after
iterate_dir() of nested overlayfs:

 [...] &ovl_i_mutex_dir_key[nested] (stack_depth=2)
 [...] ovl_i_mutex_dir_key          (stack_depth=1)
 [...] &type->i_mutex_dir_key       (stack_depth=0)

v2: specific implementation in overlayfs
v1: generic implemetnation in vfs

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/overlayfs/inode.c     | 49 ++++++++++++++++++++++++++++++++++++++++++++++++
 fs/overlayfs/overlayfs.h |  1 +
 fs/overlayfs/ovl_entry.h |  1 +
 fs/overlayfs/super.c     | 11 +++++++++++
 fs/overlayfs/util.c      |  7 +++++++
 5 files changed, 69 insertions(+)

Comments

Miklos Szeredi Dec. 5, 2016, 10 a.m. UTC | #1
On Sat, Dec 3, 2016 at 5:58 PM, Amir Goldstein <amir73il@gmail.com> wrote:

> @@ -528,6 +528,53 @@ static const struct file_operations ovl_file_operations = {
>         .open           = ovl_open,
>  };
>
> +/*
> + * It is possible to stack overlayfs instance on top of another
> + * overlayfs instance as lower layer. We need to annonate the
> + * stackable i_mutex locks according to stack level of the super
> + * block instance. An overlayfs instance can never be in stack
> + * depth 0 (there is always a real fs below it).
> + * An overlayfs instance that is not stacked over another
> + * overlayfs will use lockdep annotation name 'ovl_i_mutex_key'.
> + * An overlayfs instance that is stacked over another overlayfs
> + * will use the lockdep annotaion name ovl_i_mutex_key[nested].
> + *
> + * Note that the address ovl_i_mutex_key is the same as the address of
> + * &ovl_i_mutex_key[0], but we use the former expression to annotate
> + * the inode lock in nesting level 0 to get a nicer looking lockdep
> + * chain annotation. For example, here is a snip from

Lockdep is good, but I think we shouldn't be adding complexity into
ovl_fill_super() just for the sake of a nice looking lockdep, it's
complex enough as it is.

This patch should add a single, self contained and as simple as
possible ovl_lockdep_annotate_inode_mutex_key() function. And it
should compile away to nothing when lockdep is not enabled.

Nice looking lockdep can be a secondary goal if it doesn't complicate
the implementation.

Thanks,
Miklos
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Patch
diff mbox

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 83eaa56..04dcef4 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -528,6 +528,53 @@  static const struct file_operations ovl_file_operations = {
 	.open		= ovl_open,
 };
 
+/*
+ * It is possible to stack overlayfs instance on top of another
+ * overlayfs instance as lower layer. We need to annonate the
+ * stackable i_mutex locks according to stack level of the super
+ * block instance. An overlayfs instance can never be in stack
+ * depth 0 (there is always a real fs below it).
+ * An overlayfs instance that is not stacked over another
+ * overlayfs will use lockdep annotation name 'ovl_i_mutex_key'.
+ * An overlayfs instance that is stacked over another overlayfs
+ * will use the lockdep annotaion name ovl_i_mutex_key[nested].
+ *
+ * Note that the address ovl_i_mutex_key is the same as the address of
+ * &ovl_i_mutex_key[0], but we use the former expression to annotate
+ * the inode lock in nesting level 0 to get a nicer looking lockdep
+ * chain annotation. For example, here is a snip from
+ * /proc/lockdep_chains after dir_iterate of nested overlayfs:
+ *
+ * [...] &ovl_i_mutex_dir_key[nested] (stack_depth=2)
+ * [...] ovl_i_mutex_dir_key          (stack_depth=1)
+ * [...] &type->i_mutex_dir_key       (stack_depth=0)
+ */
+#define OVL_MAX_NESTING FILESYSTEM_MAX_STACK_DEPTH
+
+static struct lock_class_key ovl_i_mutex_key[OVL_MAX_NESTING];
+static struct lock_class_key ovl_i_mutex_dir_key[OVL_MAX_NESTING];
+
+static void ovl_lockdep_annotate_inode_mutex_key(struct inode *inode)
+{
+	int nested = ovl_nested(inode->i_sb);
+
+	if (S_ISDIR(inode->i_mode)) {
+		if (nested)
+			lockdep_set_class(&inode->i_rwsem,
+					  &ovl_i_mutex_dir_key[nested]);
+		else
+			lockdep_set_class(&inode->i_rwsem,
+					  ovl_i_mutex_dir_key);
+	} else {
+		if (nested)
+			lockdep_set_class(&inode->i_rwsem,
+					  &ovl_i_mutex_key[nested]);
+		else
+			lockdep_set_class(&inode->i_rwsem,
+					  ovl_i_mutex_key);
+	}
+}
+
 static void ovl_fill_inode(struct inode *inode, umode_t mode, dev_t rdev)
 {
 	inode->i_ino = get_next_ino();
@@ -537,6 +584,8 @@  static void ovl_fill_inode(struct inode *inode, umode_t mode, dev_t rdev)
 	inode->i_acl = inode->i_default_acl = ACL_DONT_CACHE;
 #endif
 
+	ovl_lockdep_annotate_inode_mutex_key(inode);
+
 	switch (mode & S_IFMT) {
 	case S_IFREG:
 		inode->i_op = &ovl_file_inode_operations;
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 744d744..0fdf10e 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -141,6 +141,7 @@  static inline struct inode *ovl_inode_real(struct inode *inode, bool *is_upper)
 int ovl_want_write(struct dentry *dentry);
 void ovl_drop_write(struct dentry *dentry);
 struct dentry *ovl_workdir(struct dentry *dentry);
+int ovl_nested(struct super_block *sb);
 const struct cred *ovl_override_creds(struct super_block *sb);
 struct ovl_entry *ovl_alloc_entry(unsigned int numlower);
 bool ovl_dentry_remote(struct dentry *dentry);
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index d14bca1..8bc9c54 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -23,6 +23,7 @@  struct ovl_fs {
 	struct vfsmount **lower_mnt;
 	struct dentry *workdir;
 	long namelen;
+	int nested;
 	/* pathnames of lower and upper dirs, for show_options */
 	struct ovl_config config;
 	/* creds of process who forced instantiation of super block */
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index f9a2021..8d9eadb 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -509,6 +509,17 @@  static int ovl_lower_dir(const char *name, struct path *path,
 		goto out_put;
 
 	*stack_depth = max(*stack_depth, path->mnt->mnt_sb->s_stack_depth);
+	/*
+	 * ofs->nested should mean the level of nesting of overlayfs
+	 * instances, but since FILESYSTEM_MAX_STACK_DEPTH it is not likely
+	 * to ever grow much higher than 2, use sb->s_stack_depth of lower
+	 * as a good enough approximation that guaranties:
+	 * 1. overlayfs over non-overlayfs will have ofs->nested=0
+	 * 2. a stack of several overlayfs instances will each have
+	 *    a different value of ofs->nested
+	 */
+	if (path->mnt->mnt_sb->s_magic == OVERLAYFS_SUPER_MAGIC)
+		ofs->nested = *stack_depth;
 
 	if (ovl_dentry_remote(path->dentry))
 		*remote = true;
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 952286f..21a85fc 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -32,6 +32,13 @@  struct dentry *ovl_workdir(struct dentry *dentry)
 	return ofs->workdir;
 }
 
+int ovl_nested(struct super_block *sb)
+{
+	struct ovl_fs *ofs = sb->s_fs_info;
+
+	return ofs->nested;
+}
+
 const struct cred *ovl_override_creds(struct super_block *sb)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;