Re: [PATCH 1/3] uprobes: allow put_uprobe() from non-sleepable softirq context

Oleg Nesterov
2024-09-15T14:49:34Z

On 09/09, Andrii Nakryiko wrote:
>
> Currently put_uprobe() might trigger mutex_lock()/mutex_unlock(), which
> makes it unsuitable to be called from more restricted context like softirq.
>
> Let's make put_uprobe() agnostic to the context in which it is called,
> and use work queue to defer the mutex-protected clean up steps.

...

> +static void uprobe_free_deferred(struct work_struct *work)
> +{
> + struct uprobe *uprobe = container_of(work, struct uprobe, work);
> +
> + /*
> + * If application munmap(exec_vma) before uprobe_unregister()
> + * gets called, we don't get a chance to remove uprobe from
> + * delayed_uprobe_list from remove_breakpoint(). Do it here.
> + */
> + mutex_lock(&delayed_uprobe_lock);
> + delayed_uprobe_remove(uprobe, NULL);
> + mutex_unlock(&delayed_uprobe_lock);
> +
> + kfree(uprobe);
> +}
> +
> static void uprobe_free_rcu(struct rcu_head *rcu)
> {
> struct uprobe *uprobe = container_of(rcu, struct uprobe, rcu);
>
> - kfree(uprobe);
> + INIT_WORK(&uprobe->work, uprobe_free_deferred);
> + schedule_work(&uprobe->work);
> }

This is still wrong afaics...

If put_uprobe() can be called from softirq (after the next patch), then
put_uprobe() and all other users of uprobes_treelock should use
write_lock_bh/read_lock_bh to avoid the deadlock.

To be honest... I simply can't force myself to even try to read 2/3 ;) I'll
try to do this later, but I am sure I will never like it, sorry.

Oleg.