// SPDX-License-Identifier: GPL-2.0/* * wq_stall - Test module for the workqueue stall detector. * * Deliberately creates a workqueue stall so the watchdog fires and * prints diagnostic output.  Useful for verifying that the stall * detector correctly identifies stuck workers and produces useful * backtraces. * * The stall is triggered by clearing PF_WQ_WORKER before sleeping, * which hides the worker from the concurrency manager.  A second * work item queued on the same pool then sits in the worklist with * no worker available to process it. * * After ~30s the workqueue watchdog fires: *   BUG: workqueue lockup - pool cpus=N ... * * Build: *	make -C <kernel tree> M=samples/workqueue/stall_detector modules * * Copyright (c) 2026 Meta Platforms, Inc. and affiliates. * Copyright (c) 2026 Breno Leitao <leitao@debian.org> */ #include <linux/module.h>#include <linux/workqueue.h>#include <linux/wait.h>#include <linux/atomic.h>#include <linux/sched.h> static DECLARE_WAIT_QUEUE_HEAD(stall_wq_head);static atomic_t wake_condition = ATOMIC_INIT(0);static struct work_struct stall_work1;static struct work_struct stall_work2; static void stall_work2_fn(struct work_struct *work){	pr_info("wq_stall: second work item finally ran\n");} static void stall_work1_fn(struct work_struct *work){	pr_info("wq_stall: first work item running on cpu %d\n",		raw_smp_processor_id()); 	/*	 * Queue second item while we're still counted as running	 * (pool->nr_running > 0).  Since schedule_work() on a per-CPU	 * workqueue targets raw_smp_processor_id(), item 2 lands on the	 * same pool.  __queue_work -> kick_pool -> need_more_worker()	 * sees nr_running > 0 and does NOT wake a new worker.	 */	schedule_work(&stall_work2); 	/*	 * Hide from the workqueue concurrency manager.  Without	 * PF_WQ_WORKER, schedule() won't call wq_worker_sleeping(),	 * so nr_running is never decremented and no replacement	 * worker is created.  Item 2 stays stuck in pool->worklist.	 */	current->flags &= ~PF_WQ_WORKER; 	pr_info("wq_stall: entering wait_event_idle (PF_WQ_WORKER cleared)\n");	pr_info("wq_stall: expect 'BUG: workqueue lockup' in ~30-60s\n");	wait_event_idle(stall_wq_head, atomic_read(&wake_condition) != 0); 	/* Restore so process_one_work() cleanup works correctly */	current->flags |= PF_WQ_WORKER;	pr_info("wq_stall: woke up, PF_WQ_WORKER restored\n");} static int __init wq_stall_init(void){	pr_info("wq_stall: loading\n"); 	INIT_WORK(&stall_work1, stall_work1_fn);	INIT_WORK(&stall_work2, stall_work2_fn);	schedule_work(&stall_work1); 	return 0;} static void __exit wq_stall_exit(void){	pr_info("wq_stall: unloading\n");	atomic_set(&wake_condition, 1);	wake_up(&stall_wq_head);	flush_work(&stall_work1);	flush_work(&stall_work2);	pr_info("wq_stall: all work flushed, module unloaded\n");} module_init(wq_stall_init);module_exit(wq_stall_exit); MODULE_LICENSE("GPL");MODULE_DESCRIPTION("Reproduce workqueue stall caused by PF_WQ_WORKER misuse");MODULE_AUTHOR("Breno Leitao <leitao@debian.org>");