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

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Message-ID: <6a9d765b-ce38-491d-969e-f78920d52415@roeck-us.net>
Date: Tue, 27 Jan 2026 12:31:37 -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 1/27/26 07:22, Steven Rostedt wrote:
> On Mon, 26 Jan 2026 17:32:35 -0800
> Guenter Roeck <linux@...ck-us.net> wrote:
> 
>> Still crashing, though not as often and with a slightly different backtrace.
>> I added the backtrace to the bug report @ Google.
> 
> I figured there would be other locations. A while ago I had a patch to wrap
> the checks in a "is_user_thread()" helper function[1], but Ingo had issues
> with it. It seems now it's biting us in the butt and let's see if it would
> help now. I modified it slightly.
> 
> [1] https://lore.kernel.org/linux-trace-kernel/20250425204313.616425861@goodmis.org/
> 
> If the below fixes it, I'll resend it, but now as a real bug fix.
> 

All attempts to reproduce the problem after applying the patch below failed,
so feel free to go ahead and add

Tested-by: Guenter Roeck <linux@...ck-us.net>

to the patch.

Thanks,
Guenter

> -- Steve
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index da0133524d08..5f00b5ed0f3b 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1776,6 +1776,11 @@ static __always_inline bool is_percpu_thread(void)
>   		(current->nr_cpus_allowed  == 1);
>   }
>   
> +static __always_inline bool is_user_task(struct task_struct *task)
> +{
> +	return task->mm && !(task->flags & (PF_KTHREAD | PF_USER_WORKER));
> +}
> +
>   /* Per-process atomic flags. */
>   #define PFA_NO_NEW_PRIVS		0	/* May not gain new privileges. */
>   #define PFA_SPREAD_PAGE			1	/* Spread page cache over cpuset */
> diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> index 1f6589578703..9d24b6e0c91f 100644
> --- a/kernel/events/callchain.c
> +++ b/kernel/events/callchain.c
> @@ -246,7 +246,7 @@ 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))
> +			if (!is_user_task(current))
>   				goto exit_put;
>   			regs = task_pt_regs(current);
>   		}
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index a0fa488bce84..8cca80094624 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -7460,7 +7460,7 @@ static void perf_sample_regs_user(struct perf_regs *regs_user,
>   	if (user_mode(regs)) {
>   		regs_user->abi = perf_reg_abi(current);
>   		regs_user->regs = regs;
> -	} else if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
> +	} else if (is_user_task(current)) {
>   		perf_get_regs_user(regs_user, regs);
>   	} else {
>   		regs_user->abi = PERF_SAMPLE_REGS_ABI_NONE;
> @@ -8100,7 +8100,7 @@ static u64 perf_virt_to_phys(u64 virt)
>   		 * Try IRQ-safe get_user_page_fast_only first.
>   		 * If failed, leave phys_addr as 0.
>   		 */
> -		if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
> +		if (is_user_task(current)) {
>   			struct page *p;
>   
>   			pagefault_disable();
> @@ -8215,7 +8215,7 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
>   {
>   	bool kernel = !event->attr.exclude_callchain_kernel;
>   	bool user   = !event->attr.exclude_callchain_user &&
> -		!(current->flags & (PF_KTHREAD | PF_USER_WORKER));
> +		is_user_task(current);
>   	/* Disallow cross-task user callchains. */
>   	bool crosstask = event->ctx->task && event->ctx->task != current;
>   	bool defer_user = IS_ENABLED(CONFIG_UNWIND_USER) && user &&