Re: [PATCH v2] mm/kmemleak: avoid soft lockup when scanning task stacks

[Catalin Marinas <catalin.marinas@arm.com>]

Hi Breno,

Thanks for addressing this long-standing soft lockup problem.

On Fri, Jun 12, 2026 at 08:16:07AM -0700, Breno Leitao wrote:
> +/*
> + * Briefly drop the RCU read lock to reschedule during the task stack scan.
> + * Both cursors are pinned across the gap; return false if either one was
> + * unhashed meanwhile, so the caller stops this round instead of walking a
> + * stale list.
> + */
> +static bool kmemleak_stack_scan_break(struct task_struct *g,
> +				      struct task_struct *p)
> +{
> +	bool can_cont;
> +
> +	get_task_struct(g);
> +	get_task_struct(p);
> +
> +	rcu_read_unlock();
> +	cond_resched();
> +	rcu_read_lock();
> +
> +	can_cont = pid_alive(g) && pid_alive(p);
> +
> +	put_task_struct(p);
> +	put_task_struct(g);
> +
> +	return can_cont;
> +}

While this matches rcu_lock_break(), it looks to me like we rely too
much on the internals of kernel/exit.c. Ideally this function should be
provided as an API alongside for_each_process_thread() so that we only
have the idiom in one place in case something changes in the future.

Yet anther variant below, untested. Basically, it follows the
next_tgid() or task_seq_get_next() approach (we might as well move this
to a separate function to avoid excessive indentation):

	if (kmemleak_stack_scan) {
		struct pid *pid;
		int nr = 1;

		do {
			struct task_struct *p = NULL;

			rcu_read_lock();
			pid = find_ge_pid(nr, &init_pid_ns);
			if (pid) {
				nr = pid_nr(pid) + 1;
				p = pid_task(pid, PIDTYPE_PID);
				if (p)
					get_task_struct(p);
			}
			rcu_read_unlock();

			if (p) {
				void *stack = try_get_task_stack(p);

				if (stack) {
					scan_block(stack, stack + THREAD_SIZE,
							NULL);
					put_task_stack(p);
				}
				put_task_struct(p);
			}
			cond_resched();
		} while (pid);
	}

-- 
Catalin