linux-kernel - Re: [PATCH v13 07/14] unwind_user/deferred: Make unwind deferral requests NMI-safe

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Message-ID: <20250715083506.01458000@gandalf.local.home>
Date: Tue, 15 Jul 2025 08:35:06 -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 07/14] unwind_user/deferred: Make unwind deferral
 requests NMI-safe

On Tue, 15 Jul 2025 11:09:55 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, Jul 14, 2025 at 11:11:58AM -0400, Steven Rostedt wrote:
> > On Mon, 14 Jul 2025 17:05:16 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> >   
> > > Urgh; so I hate reviewing code you're ripping out in the next patch :-(  
> > 
> > Sorry. It just happened to be developed that way. Patch 10 came about
> > to fix a bug that was triggered with the current method.  
> 
> Sure; but then you rework the series such that the bug never happened
> and reviewers don't go insane from the back and forth and possibly
> stumbling over the same bug you then fix later.
> 
> You should know this.

The bug was with actually with the next patch (#8) that uses the bitmask to
know which tracer requested a callback. Patch 8 cleared the bit after the
callbacks were called. The bug that was triggered was when the tracer set
an event to do a user space stack trace on an event that is called between
the task_work and going back to user space. It triggered an infinite loop
because the bit would get set again and trigger another task_work!

I can merge patch 8 and 10, but it still would not have affected this
patch, and would have likely led to the same confusion.

> 
> I'm going to not stare at email for some 3 weeks soon; I strongly
> suggest you take this time to fix up this series to not suffer nonsense
> like this.
> 

Sure, I'll take a deep look at your review and work on the next series to
hopefully address each of your concerns.

Thanks!

-- Steve