Re: [RESEND][PATCH 3/5] perf: Use current->flags & PF_KTHREAD|PF_USER_WORKER instead of current->mm == NULL

[Guenter Roeck <linux@roeck-us.net>]

On 1/26/26 09:18, Steven Rostedt wrote:
> On Mon, 26 Jan 2026 12:05:53 -0500
> Steven Rostedt <rostedt@kernel.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?
> 

Sure, though we had dropped the offending patch from the LTS backport, so that
will take a bit.

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);
>   		}