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

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.