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

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Message-ID: <20260126121803.4a90c959@gandalf.local.home>
Date: Mon, 26 Jan 2026 12:18:03 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Guenter Roeck <linux@...ck-us.net>
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 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?

-- 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);
 		}