On Jul 9, 2025, at 3:24 PM, NeilBrown [off-list ref] wrote:

[...]

quoted

How should the user handle -ECHILD without LOOKUP_RCU flag? Say the
following code in landlocked:

/* Try RCU walk first */
err = vfs_walk_ancestors(path, ll_cb, data, LOOKUP_RCU);

if (err == -ECHILD) {
struct path walk_path = *path;

/* reset any data changed by the walk */
reset_data(data);

/* now do ref-walk */
err = vfs_walk_ancestors(&walk_path, ll_cb, data, 0);
}

Or do you mean vfs_walk_ancestors will never return -ECHILD?
Then we need vfs_walk_ancestors to call reset_data logic, right?

It isn't clear to me that vfs_walk_ancestors() needs to return anything.
All the communication happens through walk_cb()

walk_cb() is called with a path, the data, and a "may_sleep" flag.
If it needs to sleep but may_sleep is not set, it returns "-ECHILD"
which causes the walk to restart and use refcounts.
If it wants to stop, it returns 0.
If it wants to continue, it returns 1.
If it wants a reference to the path then it can use (new)
vfs_legitimize_path() which might fail.
If it wants a reference to the path and may_sleep is true, it can use
path_get() which won't fail.

When returning -ECHILD (either because of a need to sleep or because
vfs_legitimize_path() fails), walk_cb() would reset_data().

This might actually work. 

My only concern is with vfs_legitimize_path. It is probably safer if 
we only allow taking references with may_sleep==true, so that path_get
won’t fail. In this case, we will not need walk_cb() to call 
vfs_legitimize_path. If the user want a reference, the walk_cb will 
first return -ECHILD, and call path_get when may_sleep is true. 

Does this make sense? Did I miss any cases? 

Thanks,
Song