linux-kernel - Re: [PATCH 5/8] uprobes: travers uprobe's consumer list locklessly under SRCU protection

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Message-ID: <ZqubRQ3TRsZbV9fo@krava>
Date: Thu, 1 Aug 2024 16:27:17 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: linux-trace-kernel@...r.kernel.org, peterz@...radead.org,
	oleg@...hat.com, rostedt@...dmis.org, mhiramat@...nel.org,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
	paulmck@...nel.org
Subject: Re: [PATCH 5/8] uprobes: travers uprobe's consumer list locklessly
 under SRCU protection

On Wed, Jul 31, 2024 at 02:42:53PM -0700, Andrii Nakryiko wrote:

SNIP

>  static int __copy_insn(struct address_space *mapping, struct file *filp,
>  			void *insn, int nbytes, loff_t offset)
>  {
> @@ -924,7 +901,8 @@ static bool filter_chain(struct uprobe *uprobe, struct mm_struct *mm)
>  	bool ret = false;
>  
>  	down_read(&uprobe->consumer_rwsem);
> -	for (uc = uprobe->consumers; uc; uc = uc->next) {
> +	list_for_each_entry_srcu(uc, &uprobe->consumers, cons_node,
> +				 srcu_read_lock_held(&uprobes_srcu)) {
>  		ret = consumer_filter(uc, mm);
>  		if (ret)
>  			break;
> @@ -1120,17 +1098,19 @@ void uprobe_unregister(struct uprobe *uprobe, struct uprobe_consumer *uc)
>  	int err;
>  
>  	down_write(&uprobe->register_rwsem);
> -	if (WARN_ON(!consumer_del(uprobe, uc))) {
> -		err = -ENOENT;
> -	} else {
> -		err = register_for_each_vma(uprobe, NULL);
> -		/* TODO : cant unregister? schedule a worker thread */
> -		WARN(err, "leaking uprobe due to failed unregistration");
> -	}
> +
> +	list_del_rcu(&uc->cons_node);

hum, so previous code had a check to verify that consumer is actually
registered in the uprobe, so it'd survive wrong argument while the new
code could likely do things?

> +	err = register_for_each_vma(uprobe, NULL);
> +
>  	up_write(&uprobe->register_rwsem);
>  
> -	if (!err)
> -		put_uprobe(uprobe);
> +	/* TODO : cant unregister? schedule a worker thread */
> +	if (WARN(err, "leaking uprobe due to failed unregistration"))
> +		return;
> +
> +	put_uprobe(uprobe);
> +
> +	synchronize_srcu(&uprobes_srcu);

could you comment on why it's needed in here? there's already potential
call_srcu(&uprobes_srcu, ... ) call in put_uprobe above

thanks,
jirka