Re: [PATCH v2 1/6] libfs: Re-arrange locking in offset_iterate_dir()

[Christian Brauner]

On Sat, Feb 17, 2024 at 03:23:40PM -0500, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@xxxxxxxxxx>
> 
> Liam and Matthew say that once the RCU read lock is released,
> xa_state is not safe to re-use for the next xas_find() call. But the
> RCU read lock must be released on each loop iteration so that
> dput(), which might_sleep(), can be called safely.

Fwiw, functions like this:

static struct dentry *offset_find_next(struct xa_state *xas)
{
        struct dentry *child, *found = NULL;

        rcu_read_lock();
        child = xas_next_entry(xas, U32_MAX);
        if (!child)
                goto out;
        spin_lock(&child->d_lock);
        if (simple_positive(child))
                found = dget_dlock(child);
        spin_unlock(&child->d_lock);
out:
        rcu_read_unlock();
        return found;
}

should use the new guard feature going forward imho. IOW, in the future such
helpers should be written as:

static struct dentry *offset_find_next(struct xa_state *xas)
{
        struct dentry *child, *found = NULL;

	guard(rcu)();
        child = xas_next_entry(xas, U32_MAX);
        if (!child)
		return NULL;
        spin_lock(&child->d_lock);
        if (simple_positive(child))
                found = dget_dlock(child);
        spin_unlock(&child->d_lock);
        return found;
}

which allows you to eliminate the goto and to have the guarantee that the rcu
lock is released when you return. This also works for other locks btw.