linux-kernel - Re: [PATCH perf/core 08/22] uprobes/x86: Add mapping for optimized uprobe trampolines

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Message-ID: <aA-IImjaT33ZSHcY@krava>
Date: Mon, 28 Apr 2025 15:52:34 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	x86@...nel.org, Song Liu <songliubraving@...com>,
	Yonghong Song <yhs@...com>,
	John Fastabend <john.fastabend@...il.com>,
	Hao Luo <haoluo@...gle.com>, Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Alan Maguire <alan.maguire@...cle.com>,
	David Laight <David.Laight@...lab.com>,
	Thomas Weißschuh <thomas@...ch.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH perf/core 08/22] uprobes/x86: Add mapping for optimized
 uprobe trampolines

On Sun, Apr 27, 2025 at 08:04:32PM +0200, Oleg Nesterov wrote:
> On 04/21, Jiri Olsa wrote:
> >
> > +struct uprobe_trampoline {
> > +	struct hlist_node	node;
> > +	unsigned long		vaddr;
> > +	atomic64_t		ref;
> > +};
> 
> I don't really understand the point of uprobe_trampoline->ref...
> 
> set_orig_insn/swbp_unoptimize paths don't call uprobe_trampoline_put().
> It is only called in unlikely case when swbp_optimize() fails, so perhaps
> we can kill this member and uprobe_trampoline_put() ? At least in the initial
> version.

right, we can remove that

> 
> > +static void uprobe_trampoline_put(struct uprobe_trampoline *tramp)
> > +{
> > +	if (tramp && atomic64_dec_and_test(&tramp->ref))
> > +		destroy_uprobe_trampoline(tramp);
> > +}
> 
> Why does it check tramp != NULL ?

I think some earlier version of the code could have called that with NULL,
will remove that

thanks,
jirka