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
stall_work2_fn(struct work_struct * work)36 static void stall_work2_fn(struct work_struct *work)
37 {
38 pr_info("wq_stall: second work item finally ran\n");
39 }
40
stall_work1_fn(struct work_struct * work)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
wq_stall_init(void)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
wq_stall_exit(void)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