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workqueue: Add stall detector sample module

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Add a sample module under samples/workqueue/stall_detector/ that
reproduces a workqueue stall caused by PF_WQ_WORKER misuse.  The
module queues two work items on the same per-CPU pool, then clears
PF_WQ_WORKER and sleeps in wait_event_idle(), hiding from the
concurrency manager and stalling the second work item indefinitely.

This is useful for testing the workqueue watchdog stall diagnostics.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Breno Leitao
2026-03-05 08:15:41 -08:00
committed by Tejun Heo
parent 8823eaef45
commit 9e83d5104a
2 changed files with 99 additions and 0 deletions
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1
samples/workqueue/stall_detector/Makefile Normal file
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obj-m += wq_stall.o

98
samples/workqueue/stall_detector/wq_stall.c Normal file
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// 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>");