vfs: make d_path() get the root path under RCU

This avoids the spinlocks and refcounts in the d_path() sequence too
(used by /proc and various other entities).  See commit 8b19e34188a3 for
the equivalent getcwd() system call path.

And unlike getcwd(), d_path() doesn't copy the result to user space, so
I don't need to fear _that_ particular bug happening again.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/dcache.c b/fs/dcache.c
index 91e551b..dddc67f 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2869,6 +2869,16 @@ static int prepend_unreachable(char **buffer, int *buflen)
        return prepend(buffer, buflen, "(unreachable)", 13);
 }
 
+static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
+{
+       unsigned seq;
+
+       do {
+               seq = read_seqcount_begin(&fs->seq);
+               *root = fs->root;
+       } while (read_seqcount_retry(&fs->seq, seq));
+}
+
 /**
  * d_path - return the path of a dentry
  * @path: path to report
@@ -2901,13 +2911,15 @@ char *d_path(const struct path *path, char *buf, int buflen)
        if (path->dentry->d_op && path->dentry->d_op->d_dname)
                return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
 
-       get_fs_root(current->fs, &root);
+       rcu_read_lock();
+       get_fs_root_rcu(current->fs, &root);
        br_read_lock(&vfsmount_lock);
        error = path_with_deleted(path, &root, &res, &buflen);
        br_read_unlock(&vfsmount_lock);
+       rcu_read_unlock();
+
        if (error < 0)
                res = ERR_PTR(error);
-       path_put(&root);
        return res;
 }
 EXPORT_SYMBOL(d_path);