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

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Message-ID: <20250715132016.409b1082@batman.local.home>
Date: Tue, 15 Jul 2025 13:20:16 -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 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.

By having a WARN_ON(), we need to make perf aware of this limitation
too. Do we want to do that?

-- Steve