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.
