@@ -31,6 +31,8 @@ prototypes::
struct vfsmount *(*d_automount)(struct path *path);
int (*d_manage)(const struct path *, bool);
struct dentry *(*d_real)(struct dentry *, enum d_real_type type);
+ bool (*d_unalias_trylock)(const struct dentry *);
+ void (*d_unalias_unlock)(const struct dentry *);
locking rules:
@@ -50,6 +52,8 @@ d_dname: no no no no
d_automount: no no yes no
d_manage: no no yes (ref-walk) maybe
d_real no no yes no
+d_unalias_trylock yes no no no
+d_unalias_unlock yes no no no
================== =========== ======== ============== ========
inode_operations
@@ -1265,6 +1265,8 @@ defined:
struct vfsmount *(*d_automount)(struct path *);
int (*d_manage)(const struct path *, bool);
struct dentry *(*d_real)(struct dentry *, enum d_real_type type);
+ bool (*d_unalias_trylock)(const struct dentry *);
+ void (*d_unalias_unlock)(const struct dentry *);
};
``d_revalidate``
@@ -1428,6 +1430,25 @@ defined:
For non-regular files, the 'dentry' argument is returned.
+``d_unalias_trylock``
+ if present, will be called by d_splice_alias() before moving a
+ preexisting attached alias. Returning false prevents __d_move(),
+ making d_splice_alias() fail with -ESTALE.
+
+ Rationale: setting FS_RENAME_DOES_D_MOVE will prevent d_move()
+ and d_exchange() calls from the outside of filesystem methods;
+ however, it does not guarantee that attached dentries won't
+ be renamed or moved by d_splice_alias() finding a preexisting
+ alias for a directory inode. Normally we would not care;
+ however, something that wants to stabilize the entire path to
+ root over a blocking operation might need that. See 9p for one
+ (and hopefully only) example.
+
+``d_unalias_unlock``
+ should be paired with ``d_unalias_trylock``; that one is called after
+ __d_move() call in __d_unalias().
+
+
Each dentry has a pointer to its parent dentry, as well as a hash list
of child dentries. Child dentries are basically like files in a
directory.
@@ -202,7 +202,7 @@ static inline struct v9fs_session_info *v9fs_inode2v9ses(struct inode *inode)
return inode->i_sb->s_fs_info;
}
-static inline struct v9fs_session_info *v9fs_dentry2v9ses(struct dentry *dentry)
+static inline struct v9fs_session_info *v9fs_dentry2v9ses(const struct dentry *dentry)
{
return dentry->d_sb->s_fs_info;
}
@@ -105,14 +105,30 @@ static int v9fs_lookup_revalidate(struct inode *dir, const struct qstr *name,
return __v9fs_lookup_revalidate(dentry, flags);
}
+static bool v9fs_dentry_unalias_trylock(const struct dentry *dentry)
+{
+ struct v9fs_session_info *v9ses = v9fs_dentry2v9ses(dentry);
+ return down_write_trylock(&v9ses->rename_sem);
+}
+
+static void v9fs_dentry_unalias_unlock(const struct dentry *dentry)
+{
+ struct v9fs_session_info *v9ses = v9fs_dentry2v9ses(dentry);
+ up_write(&v9ses->rename_sem);
+}
+
const struct dentry_operations v9fs_cached_dentry_operations = {
.d_revalidate = v9fs_lookup_revalidate,
.d_weak_revalidate = __v9fs_lookup_revalidate,
.d_delete = v9fs_cached_dentry_delete,
.d_release = v9fs_dentry_release,
+ .d_unalias_trylock = v9fs_dentry_unalias_trylock,
+ .d_unalias_unlock = v9fs_dentry_unalias_unlock,
};
const struct dentry_operations v9fs_dentry_operations = {
.d_delete = always_delete_dentry,
.d_release = v9fs_dentry_release,
+ .d_unalias_trylock = v9fs_dentry_unalias_trylock,
+ .d_unalias_unlock = v9fs_dentry_unalias_unlock,
};
@@ -2961,7 +2961,12 @@ static int __d_unalias(struct dentry *dentry, struct dentry *alias)
goto out_err;
m2 = &alias->d_parent->d_inode->i_rwsem;
out_unalias:
+ if (alias->d_op->d_unalias_trylock &&
+ !alias->d_op->d_unalias_trylock(alias))
+ goto out_err;
__d_move(alias, dentry, false);
+ if (alias->d_op->d_unalias_unlock)
+ alias->d_op->d_unalias_unlock(alias);
ret = 0;
out_err:
if (m2)
@@ -159,6 +159,8 @@ struct dentry_operations {
struct vfsmount *(*d_automount)(struct path *);
int (*d_manage)(const struct path *, bool);
struct dentry *(*d_real)(struct dentry *, enum d_real_type type);
+ bool (*d_unalias_trylock)(const struct dentry *);
+ void (*d_unalias_unlock)(const struct dentry *);
} ____cacheline_aligned;
/*
9p wants to be able to build a path from given dentry to fs root and keep it valid over a blocking operation. ->s_vfs_rename_mutex would be a natural candidate, but there are places where we need that and where we have no way to tell if ->s_vfs_rename_mutex is already held deeper in callchain. Moreover, it's only held for cross-directory renames; name changes within the same directory happen without it. Solution: * have d_move() done in ->rename() rather than in its caller * maintain a 9p-private rwsem (per-filesystem) * hold it exclusive over the relevant part of ->rename() * hold it shared over the places where we want the path. That almost works. FS_RENAME_DOES_D_MOVE is enough to put all d_move() and d_exchange() calls under filesystem's control. However, there's also __d_unalias(), which isn't covered by any of that. If ->lookup() hits a directory inode with preexisting dentry elsewhere (due to e.g. rename done on server behind our back), d_splice_alias() called by ->lookup() will move/rename that alias. Add a couple of optional methods, so that __d_unalias() would do if alias->d_op->d_unalias_trylock != NULL if (!alias->d_op->d_unalias_trylock(alias)) fail (resulting in -ESTALE from lookup) __d_move(...) if alias->d_op->d_unalias_unlock != NULL alias->d_unalias_unlock(alias) where it currently does __d_move(). 9p instances do down_write_trylock() and up_write() of ->rename_mutex. Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> --- Documentation/filesystems/locking.rst | 4 ++++ Documentation/filesystems/vfs.rst | 21 +++++++++++++++++++++ fs/9p/v9fs.h | 2 +- fs/9p/vfs_dentry.c | 16 ++++++++++++++++ fs/dcache.c | 5 +++++ include/linux/dcache.h | 2 ++ 6 files changed, 49 insertions(+), 1 deletion(-)