linux-kernel - Re: [PATCH v13 10/14] unwind: Clear unwind

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Message-ID: <20250715190445.GG4105545@noisy.programming.kicks-ass.net>
Date: Tue, 15 Jul 2025 21:04:45 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Steven Rostedt <rostedt@...nel.org>, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
	x86@...nel.org, Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrii Nakryiko <andrii@...nel.org>,
	Indu Bhagat <indu.bhagat@...cle.com>,
	"Jose E. Marchesi" <jemarch@....org>,
	Beau Belgrave <beaub@...ux.microsoft.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>, Florian Weimer <fweimer@...hat.com>,
	Sam James <sam@...too.org>
Subject: Re: [PATCH v13 10/14] unwind: Clear unwind_mask on exit back to user
 space

On Tue, Jul 15, 2025 at 02:06:50PM -0400, Steven Rostedt wrote:
> On Tue, 15 Jul 2025 08:49:32 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > > >   *
> > > > - * Return: 1 if the the callback was already queued.
> > > > - *         0 if the callback successfully was queued.
> > > > + * Return: 0 if the callback successfully was queued.
> > > > + *         UNWIND_ALREADY_PENDING if the the callback was already queued.
> > > > + *         UNWIND_ALREADY_EXECUTED if the callback was already called
> > > > + *                (and will not be called again)
> > > >   *         Negative if there's an error.
> > > >   *         @cookie holds the cookie of the first request by any user
> > > >   */    
> > > 
> > > Lots of babbling in the Changelog, but no real elucidation as to why you
> > > need this second return value.
> > > 
> > > AFAICT it serves no real purpose; the users of this function should not
> > > care. The only difference is that the unwind reference (your cookie)
> > > becomes a backward reference instead of a forward reference. But why
> > > would anybody care?  
> > 
> > Older versions of the code required it. I think I can remove it now.
> 
> Ah it is still used in the perf code:
> 
> perf_callchain() has:
> 
>         if (defer_user) {
>                 int ret = deferred_request(event);
>                 if (!ret)
>                         local_inc(&event->ctx->nr_no_switch_fast);
>                 else if (ret < 0)
>                         defer_user = false;
>         }
> 
> Where deferred_requests() is as static function that returns the result
> of the unwind request. If it is zero, it means the callback will be
> called, if it is greater than zero it means it has already been called,
> and negative is an error (and use the old method).
> 
> It looks like when the callback is called it expects nr_no_switch_fast
> to be incremented and it will decrement it. This is directly from
> Josh's patch and I don't know perf well enough to know if that update
> to nr_no_switch_fast is needed.
> 
> If it's not needed, we can just return 0 on success and negative on
> failure. What do you think?

I'm yet again confused. I don't see this code differentiate between 1
and 2 return values (those PENDING and EXECUTED).

Anyway, fundamentally I don't think there is a problem with backward
references as opposed to the normal forward references.

So leave it out for now.