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

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Message-ID: <20210129162438.GC8912@worktop.programming.kicks-ass.net>
Date:   Fri, 29 Jan 2021 17:24:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Nikolay Borisov <nborisov@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>, 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 10:59:52AM -0500, Steven Rostedt wrote:
> On Fri, 29 Jan 2021 22:40:11 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > > So what, they can all happen with random locks held. Marking them as NMI
> > > enables a whole bunch of sanity checks that are entirely appropriate.  
> > 
> > How about introducing an idea of Asynchronous NMI (ANMI) and Synchronous
> > NMI (SNMI)? kprobes and ftrace is synchronously called and can be controlled
> > (we can expect the context) but ANMI may be caused by asynchronous 
> > hardware events on any context.
> > 
> > If we can distinguish those 2 NMIs on preempt count, bpf people can easily
> > avoid the inevitable situation.
> 
> I don't like the name NMI IN SNMI, because they are not NMIs. They are
> actually more like kernel exceptions. Even page faults in the kernel is
> similar to a kprobe breakpoint or ftrace. It can happen anywhere, with any
> lock held. Perhaps we need a kernel exception context? Which by definition
> is synchronous.

What problem are you trying to solve? AFAICT all these contexts have the
same restrictions, why try and muck about with different names for the
same thing?