vfs: make getcwd() get the root and pwd path under rcu
This allows us to skip all the crazy spinlocks and reference count
updates, and instead use the fs sequence read-lock to get an atomic
snapshot of the root and cwd information.
We might want to make the rule that "prepend_path()" is always called
with the RCU lock held, but the RCU lock nests fine and this is the
minimal fix.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/dcache.c b/fs/dcache.c
index 4df68e2..99d4d72 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -3015,15 +3015,16 @@
return ERR_PTR(-ENAMETOOLONG);
}
-static inline void get_fs_root_and_pwd(struct fs_struct *fs, struct path *root,
- struct path *pwd)
+static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path *root,
+ struct path *pwd)
{
- spin_lock(&fs->lock);
- *root = fs->root;
- path_get(root);
- *pwd = fs->pwd;
- path_get(pwd);
- spin_unlock(&fs->lock);
+ unsigned seq;
+
+ do {
+ seq = read_seqcount_begin(&fs->seq);
+ *root = fs->root;
+ *pwd = fs->pwd;
+ } while (read_seqcount_retry(&fs->seq, seq));
}
/*
@@ -3053,7 +3054,8 @@
if (!page)
return -ENOMEM;
- get_fs_root_and_pwd(current->fs, &root, &pwd);
+ rcu_read_lock();
+ get_fs_root_and_pwd_rcu(current->fs, &root, &pwd);
error = -ENOENT;
br_read_lock(&vfsmount_lock);
@@ -3088,8 +3090,7 @@
}
out:
- path_put(&pwd);
- path_put(&root);
+ rcu_read_unlock();
free_page((unsigned long) page);
return error;
}