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SubjectRe: [PATCH v3] workqueue: Fix WARN_ON_ONCE() triggers in worker_enter_idle()
Hello,

I updated the comment and description and applied the patch to wq/for-6.5.

Thanks.

From c8f6219be2e58d7f676935ae90b64abef5d0966a Mon Sep 17 00:00:00 2001
From: Zqiang <qiang.zhang1211@gmail.com>
Date: Wed, 24 May 2023 11:53:39 +0800
Subject: [PATCH] workqueue: Fix WARN_ON_ONCE() triggers in worker_enter_idle()

Currently, pool->nr_running can be modified from timer tick, that means the
timer tick can run nested inside a not-irq-protected section that's in the
process of modifying nr_running. Consider the following scenario:

CPU0
kworker/0:2 (events)
worker_clr_flags(worker, WORKER_PREP | WORKER_REBOUND);
->pool->nr_running++; (1)

process_one_work()
->worker->current_func(work);
->schedule()
->wq_worker_sleeping()
->worker->sleeping = 1;
->pool->nr_running--; (0)
....
->wq_worker_running()
....
CPU0 by interrupt:
wq_worker_tick()
->worker_set_flags(worker, WORKER_CPU_INTENSIVE);
->pool->nr_running--; (-1)
->worker->flags |= WORKER_CPU_INTENSIVE;
....
->if (!(worker->flags & WORKER_NOT_RUNNING))
->pool->nr_running++; (will not execute)
->worker->sleeping = 0;
....
->worker_clr_flags(worker, WORKER_CPU_INTENSIVE);
->pool->nr_running++; (0)
....
worker_set_flags(worker, WORKER_PREP);
->pool->nr_running--; (-1)
....
worker_enter_idle()
->WARN_ON_ONCE(pool->nr_workers == pool->nr_idle && pool->nr_running);

if the nr_workers is equal to nr_idle, due to the nr_running is not zero,
will trigger WARN_ON_ONCE().

[ 2.460602] WARNING: CPU: 0 PID: 63 at kernel/workqueue.c:1999 worker_enter_idle+0xb2/0xc0
[ 2.462163] Modules linked in:
[ 2.463401] CPU: 0 PID: 63 Comm: kworker/0:2 Not tainted 6.4.0-rc2-next-20230519 #1
[ 2.463771] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014
[ 2.465127] Workqueue: 0x0 (events)
[ 2.465678] RIP: 0010:worker_enter_idle+0xb2/0xc0
...
[ 2.472614] Call Trace:
[ 2.473152] <TASK>
[ 2.474182] worker_thread+0x71/0x430
[ 2.474992] ? _raw_spin_unlock_irqrestore+0x28/0x50
[ 2.475263] kthread+0x103/0x120
[ 2.475493] ? __pfx_worker_thread+0x10/0x10
[ 2.476355] ? __pfx_kthread+0x10/0x10
[ 2.476635] ret_from_fork+0x2c/0x50
[ 2.477051] </TASK>

This commit therefore add the check of worker->sleeping in wq_worker_tick(),
if the worker->sleeping is not zero, directly return.

tj: Updated comment and description.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Closes: https://qa-reports.linaro.org/lkft/linux-next-master/build/next-20230519/testrun/17078554/suite/boot/test/clang-nightly-lkftconfig/log
Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/workqueue.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index ee16ddb0647c..3ad6806c7161 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1051,7 +1051,7 @@ void wq_worker_running(struct task_struct *task)
{
struct worker *worker = kthread_data(task);

- if (!worker->sleeping)
+ if (!READ_ONCE(worker->sleeping))
return;

/*
@@ -1071,7 +1071,7 @@ void wq_worker_running(struct task_struct *task)
*/
worker->current_at = worker->task->se.sum_exec_runtime;

- worker->sleeping = 0;
+ WRITE_ONCE(worker->sleeping, 0);
}

/**
@@ -1097,10 +1097,10 @@ void wq_worker_sleeping(struct task_struct *task)
pool = worker->pool;

/* Return if preempted before wq_worker_running() was reached */
- if (worker->sleeping)
+ if (READ_ONCE(worker->sleeping))
return;

- worker->sleeping = 1;
+ WRITE_ONCE(worker->sleeping, 1);
raw_spin_lock_irq(&pool->lock);

/*
@@ -1143,8 +1143,15 @@ void wq_worker_tick(struct task_struct *task)
* If the current worker is concurrency managed and hogged the CPU for
* longer than wq_cpu_intensive_thresh_us, it's automatically marked
* CPU_INTENSIVE to avoid stalling other concurrency-managed work items.
+ *
+ * Set @worker->sleeping means that @worker is in the process of
+ * switching out voluntarily and won't be contributing to
+ * @pool->nr_running until it wakes up. As wq_worker_sleeping() also
+ * decrements ->nr_running, setting CPU_INTENSIVE here can lead to
+ * double decrements. The task is releasing the CPU anyway. Let's skip.
+ * We probably want to make this prettier in the future.
*/
- if ((worker->flags & WORKER_NOT_RUNNING) ||
+ if ((worker->flags & WORKER_NOT_RUNNING) || READ_ONCE(worker->sleeping) ||
worker->task->se.sum_exec_runtime - worker->current_at <
wq_cpu_intensive_thresh_us * NSEC_PER_USEC)
return;
--
2.40.1
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