linux-kernel - Re: [RESEND][PATCH 3/5] perf: Use current->flags & PF_KTHREAD|PF_USER

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Message-ID: <27f9a957-67c7-4c88-b320-0f8725e9b03d@roeck-us.net>
Date: Mon, 26 Jan 2026 17:32:35 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 linux-perf-users@...r.kernel.org, Masami Hiramatsu <mhiramat@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RESEND][PATCH 3/5] perf: Use current->flags &
 PF_KTHREAD|PF_USER_WORKER instead of current->mm == NULL

On 1/26/26 09:18, Steven Rostedt wrote:
> On Mon, 26 Jan 2026 12:05:53 -0500
> Steven Rostedt <rostedt@...nel.org> wrote:
> 
>> I guess we need to also test for !current->mm because the flags set for an
>> exiting task is done when we can still do callchains. Thus, the only way to
>> know if it is safe to do a callchain when a task is exiting is via task->mm
>> and not task->flags :-/
> 
> Can you test this patch?
> 

Still crashing, though not as often and with a slightly different backtrace.
I added the backtrace to the bug report @ Google.

Guenter

> -- Steve
> 
> diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> index 1f6589578703..c82d61d73bd8 100644
> --- a/kernel/events/callchain.c
> +++ b/kernel/events/callchain.c
> @@ -246,7 +246,14 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
>   
>   	if (user && !crosstask) {
>   		if (!user_mode(regs)) {
> -			if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
> +			/*
> +			 * Testing current->mm is not enough as some kernel threads
> +			 * may have one set. But testing the flags is not enough
> +			 * either as this can be called after a user task
> +			 * frees its mm just before it exits.
> +			 */
> +			if (!current->mm ||
> +			    (current->flags & (PF_KTHREAD | PF_USER_WORKER)))
>   				goto exit_put;
>   			regs = task_pt_regs(current);
>   		}