linux-kernel - Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi

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Message-ID: <YBUYsFlxjsQxuvfB@hirez.programming.kicks-ass.net>
Date:   Sat, 30 Jan 2021 09:28:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Nikolay Borisov <nborisov@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter()
 with nmi_enter()")

On Fri, Jan 29, 2021 at 04:24:54PM -0500, Steven Rostedt wrote:
> Specifically, kprobe and ftrace callbacks may have this:
> 
> 	if (in_nmi())
> 		return;
> 
> 	raw_spin_lock_irqsave(&lock, flags);
> 	[..]
> 	raw_spin_unlock_irqrestore(&lock, flags);
> 
> Which is totally fine to have,

Why? There's a distinct lack of explaining here.

Note that we ripped out all such dodgy locking from kretprobes.