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

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Message-ID: <20250714150516.GE4105545@noisy.programming.kicks-ass.net>
Date: Mon, 14 Jul 2025 17:05:16 +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 07/14] unwind_user/deferred: Make unwind deferral
 requests NMI-safe

On Mon, Jul 14, 2025 at 10:19:19AM -0400, Steven Rostedt wrote:
> On Mon, 14 Jul 2025 15:29:36 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > +#ifdef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
> > +#define UNWIND_NMI_SAFE 1
> > +static inline bool try_assign_cnt(struct unwind_task_info *info, u32 cnt)
> > +{
> > +	u32 zero = 0;
> > +	return try_cmpxchg(&info->id.cnt, &zero, cnt);
> > +}
> > +static inline bool test_and_set_pending(struct unwind_task_info *info)
> > +{
> > +	return info->pending || cmpxchg_local(&info->pending, 0, 1);
> > +}
> > +#el
> 
> Patch 10 moves the pending bit into the unwind_mask as it needs to be
> in sync with the different tracer requests. I'm not sure how this
> change will interact with that.

Urgh; so I hate reviewing code you're ripping out in the next patch :-(

Let me go stare at that.