linux-kernel - Re: [PATCH v10 04/14] unwind_user/deferred: Add unwind_deferred

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Message-ID: <20250618140247.GQ1613376@noisy.programming.kicks-ass.net>
Date: Wed, 18 Jun 2025 16:02:47 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: 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>
Subject: Re: [PATCH v10 04/14] unwind_user/deferred: Add
 unwind_deferred_trace()

On Tue, Jun 10, 2025 at 08:54:25PM -0400, Steven Rostedt wrote:
> +/**
> + * unwind_deferred_trace - Produce a user stacktrace in faultable context
> + * @trace: The descriptor that will store the user stacktrace
> + *
> + * This must be called in a known faultable context (usually when entering
> + * or exiting user space). Depending on the available implementations
> + * the @trace will be loaded with the addresses of the user space stacktrace
> + * if it can be found.

I am confused -- why would we ever want to call this on exiting
user-space, or rather kernel entry?

I thought the whole point was to request a user trace while in-kernel,
and defer that to return-to-user.