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

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Message-ID: <20250715142610.2e1bb341@batman.local.home>
Date: Tue, 15 Jul 2025 14:26:10 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.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, 15 Jul 2025 14:06:50 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> > > > + * 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);

Hmm, I guess this could work if it returned non zero for both already
queued and already executed. So it doesn't need to be two different
values.

-- Steve

>                 else if (ret < 0)
>                         defer_user = false;
>         }