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

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Message-ID: <20250715190722.GH4105545@noisy.programming.kicks-ass.net>
Date: Tue, 15 Jul 2025 21:07:22 +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 01:20:16PM -0400, Steven Rostedt wrote:
> On Tue, 15 Jul 2025 12:29:12 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > @@ -170,41 +193,62 @@ static void unwind_deferred_task_work(st
> >  int unwind_deferred_request(struct unwind_work *work, u64 *cookie)
> >  {
> >  	struct unwind_task_info *info = &current->unwind_info;
> > -	int ret;
> > +	unsigned long bits, mask;
> > +	int bit, ret;
> >  
> >  	*cookie = 0;
> >  
> > -	if (WARN_ON_ONCE(in_nmi()))
> > -		return -EINVAL;
> > -
> >  	if ((current->flags & (PF_KTHREAD | PF_EXITING)) ||
> >  	    !user_mode(task_pt_regs(current)))
> >  		return -EINVAL;
> >  
> > +	/* NMI requires having safe cmpxchg operations */
> > +	if (WARN_ON_ONCE(!UNWIND_NMI_SAFE && in_nmi()))
> > +		return -EINVAL;
> 
> I don't think we want to have a WARN_ON() here as the perf series tries
> to first do the deferred unwinding and if that fails, it will go back
> to it's old method.

The thing is, I don't think we have an architecture that supports NMIs
and does not have NMI safe cmpxchg. And if we do have one such -- I
don't think it has perf; perf very much assumes cmpxchg is NMI safe.

Calling this from NMI context and not having an NMI safe cmpxchg is very
much a dodgy use case. Please leave the WARN, if it ever triggers, we'll
look at who manages and deal with it then.