Re: [RFC PATCH 2/2] workqueue: Fix unbind_workers() VS wq_worker_sleeping() race

[Lai Jiangshan]

On Tue, Nov 30, 2021 at 8:06 AM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:
>
> At CPU-hotplug time, unbind_workers() may preempt a worker while it is
> going to sleep. In that case the following scenario can happen:
>
>     unbind_workers()                     wq_worker_sleeping()
>     --------------                      -------------------
>                                       if (worker->flags & WORKER_NOT_RUNNING)
>                                           return;
>                                       //PREEMPTED by unbind_workers
>     worker->flags |= WORKER_UNBOUND;
>     [...]
>     atomic_set(&pool->nr_running, 0);
>     //resume to worker
>                                        atomic_dec_and_test(&pool->nr_running);
>
> After unbind_worker() resets pool->nr_running, the value is expected to
> remain 0 until the pool ever gets rebound in case cpu_up() is called on
> the target CPU in the future. But here the race leaves pool->nr_running
> with a value of -1, triggering the following warning when the worker goes
> idle:
>
>         WARNING: CPU: 3 PID: 34 at kernel/workqueue.c:1823 worker_enter_idle+0x95/0xc0
>         Modules linked in:
>         CPU: 3 PID: 34 Comm: kworker/3:0 Not tainted 5.16.0-rc1+ #34
>         Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba527-rebuilt.opensuse.org 04/01/2014
>         Workqueue:  0x0 (rcu_par_gp)
>         RIP: 0010:worker_enter_idle+0x95/0xc0
>         Code: 04 85 f8 ff ff ff 39 c1 7f 09 48 8b 43 50 48 85 c0 74 1b 83 e2 04 75 99 8b 43 34 39 43 30 75 91 8b 83 00 03 00 00 85 c0 74 87 <0f> 0b 5b c3 48 8b 35 70 f1 37 01 48 8d 7b 48 48 81 c6 e0 93  0
>         RSP: 0000:ffff9b7680277ed0 EFLAGS: 00010086
>         RAX: 00000000ffffffff RBX: ffff93465eae9c00 RCX: 0000000000000000
>         RDX: 0000000000000000 RSI: ffff9346418a0000 RDI: ffff934641057140
>         RBP: ffff934641057170 R08: 0000000000000001 R09: ffff9346418a0080
>         R10: ffff9b768027fdf0 R11: 0000000000002400 R12: ffff93465eae9c20
>         R13: ffff93465eae9c20 R14: ffff93465eae9c70 R15: ffff934641057140
>         FS:  0000000000000000(0000) GS:ffff93465eac0000(0000) knlGS:0000000000000000
>         CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>         CR2: 0000000000000000 CR3: 000000001cc0c000 CR4: 00000000000006e0
>         DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>         DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>         Call Trace:
>           <TASK>
>           worker_thread+0x89/0x3d0
>           ? process_one_work+0x400/0x400
>           kthread+0x162/0x190
>           ? set_kthread_struct+0x40/0x40
>           ret_from_fork+0x22/0x30
>           </TASK>
>
> Also due to this incorrect "nr_running == -1", all sorts of hazards can
> happen, starting with queued works being ignored because no workers are
> awaken at insert_work() time.
>
> Fix this with checking again the worker flags while pool->lock is locked.
>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>


Fixes: b945efcdd07d ("sched: Remove pointless preemption disable in
sched_submit_work()")
Reviewed-by: Lai Jiangshan <jiangshanlai@xxxxxxxxx>

It was my fault for not reviewing b945efcdd07d carefully enough.

> ---
>  kernel/workqueue.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 5094573e8b45..5557d19ea81c 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -912,6 +912,16 @@ void wq_worker_sleeping(struct task_struct *task)
>         worker->sleeping = 1;
>         raw_spin_lock_irq(&pool->lock);
>
> +       /*
> +        * Recheck in case unbind_workers() preempted us. We don't
> +        * want to decrement nr_running after the worker is unbound
> +        * and nr_running has been reset.
> +        */
> +       if (worker->flags & WORKER_NOT_RUNNING) {
> +               raw_spin_unlock_irq(&pool->lock);
> +               return;
> +       }
> +
>         /*
>          * The counterpart of the following dec_and_test, implied mb,
>          * worklist not empty test sequence is in insert_work().
> --
> 2.25.1
>