1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * wq_stall - Test module for the workqueue stall detector. 4 * 5 * Deliberately creates a workqueue stall so the watchdog fires and 6 * prints diagnostic output. Useful for verifying that the stall 7 * detector correctly identifies stuck workers and produces useful 8 * backtraces. 9 * 10 * The stall is triggered by clearing PF_WQ_WORKER before sleeping, 11 * which hides the worker from the concurrency manager. A second 12 * work item queued on the same pool then sits in the worklist with 13 * no worker available to process it. 14 * 15 * After ~30s the workqueue watchdog fires: 16 * BUG: workqueue lockup - pool cpus=N ... 17 * 18 * Build: 19 * make -C <kernel tree> M=samples/workqueue/stall_detector modules 20 * 21 * Copyright (c) 2026 Meta Platforms, Inc. and affiliates. 22 * Copyright (c) 2026 Breno Leitao <leitao@debian.org> 23 */ 24 25 #include <linux/module.h> 26 #include <linux/workqueue.h> 27 #include <linux/wait.h> 28 #include <linux/atomic.h> 29 #include <linux/sched.h> 30 31 static DECLARE_WAIT_QUEUE_HEAD(stall_wq_head); 32 static atomic_t wake_condition = ATOMIC_INIT(0); 33 static struct work_struct stall_work1; 34 static struct work_struct stall_work2; 35 36 static void stall_work2_fn(struct work_struct *work) 37 { 38 pr_info("wq_stall: second work item finally ran\n"); 39 } 40 41 static void stall_work1_fn(struct work_struct *work) 42 { 43 pr_info("wq_stall: first work item running on cpu %d\n", 44 raw_smp_processor_id()); 45 46 /* 47 * Queue second item while we're still counted as running 48 * (pool->nr_running > 0). Since schedule_work() on a per-CPU 49 * workqueue targets raw_smp_processor_id(), item 2 lands on the 50 * same pool. __queue_work -> kick_pool -> need_more_worker() 51 * sees nr_running > 0 and does NOT wake a new worker. 52 */ 53 schedule_work(&stall_work2); 54 55 /* 56 * Hide from the workqueue concurrency manager. Without 57 * PF_WQ_WORKER, schedule() won't call wq_worker_sleeping(), 58 * so nr_running is never decremented and no replacement 59 * worker is created. Item 2 stays stuck in pool->worklist. 60 */ 61 current->flags &= ~PF_WQ_WORKER; 62 63 pr_info("wq_stall: entering wait_event_idle (PF_WQ_WORKER cleared)\n"); 64 pr_info("wq_stall: expect 'BUG: workqueue lockup' in ~30-60s\n"); 65 wait_event_idle(stall_wq_head, atomic_read(&wake_condition) != 0); 66 67 /* Restore so process_one_work() cleanup works correctly */ 68 current->flags |= PF_WQ_WORKER; 69 pr_info("wq_stall: woke up, PF_WQ_WORKER restored\n"); 70 } 71 72 static int __init wq_stall_init(void) 73 { 74 pr_info("wq_stall: loading\n"); 75 76 INIT_WORK(&stall_work1, stall_work1_fn); 77 INIT_WORK(&stall_work2, stall_work2_fn); 78 schedule_work(&stall_work1); 79 80 return 0; 81 } 82 83 static void __exit wq_stall_exit(void) 84 { 85 pr_info("wq_stall: unloading\n"); 86 atomic_set(&wake_condition, 1); 87 wake_up(&stall_wq_head); 88 flush_work(&stall_work1); 89 flush_work(&stall_work2); 90 pr_info("wq_stall: all work flushed, module unloaded\n"); 91 } 92 93 module_init(wq_stall_init); 94 module_exit(wq_stall_exit); 95 96 MODULE_LICENSE("GPL"); 97 MODULE_DESCRIPTION("Reproduce workqueue stall caused by PF_WQ_WORKER misuse"); 98 MODULE_AUTHOR("Breno Leitao <leitao@debian.org>"); 99