Re: [RFC PATCH 1/2] workqueue: Fix unbind_workers() VS wq_worker_running() race

[Lai Jiangshan <jiangshanlai@gmail.com>]

On Tue, Nov 30, 2021 at 8:06 AM Frederic Weisbecker <frederic@kernel.org> wrote:
>
> At CPU-hotplug time, unbind_worker() may preempt a worker while it is
> waking up. In that case the following scenario can happen:
>
>         unbind_workers()                     wq_worker_running()
>         --------------                      -------------------
>                                       if (!(worker->flags & WORKER_NOT_RUNNING))
>                                           //PREEMPTED by unbind_workers
>         worker->flags |= WORKER_UNBOUND;
>         [...]
>         atomic_set(&pool->nr_running, 0);
>         //resume to worker
>                                               atomic_inc(&worker->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", further queued work may
> end up not being served, because no worker is awaken at work insert time.
> This raises rcutorture writer stalls for example.
>
> Fix this with disabling preemption in the right place in
> wq_worker_running().
>
> It's worth noting that if the worker migrates and runs concurrently with
> unbind_workers(), it is guaranteed to see the WORKER_UNBOUND flag update
> due to set_cpus_allowed_ptr() acquiring/releasing rq->lock.
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Fixes: 6d25be5782e4 ("sched/core, workqueues: Distangle worker
accounting from rq lock")

Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>

> Cc: Paul E. McKenney <paulmck@kernel.org>
> ---
>  kernel/workqueue.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 332361cf215f..5094573e8b45 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -868,8 +868,17 @@ void wq_worker_running(struct task_struct *task)
>
>         if (!worker->sleeping)
>                 return;
> +
> +       /*
> +        * If preempted by unbind_workers() between the WORKER_NOT_RUNNING check
> +        * and the nr_running increment below, we may ruin the nr_running reset
> +        * and leave with an unexpected pool->nr_running == 1 on the newly unbound
> +        * pool. Protect against such race.
> +        */
> +       preempt_disable();
>         if (!(worker->flags & WORKER_NOT_RUNNING))
>                 atomic_inc(&worker->pool->nr_running);
> +       preempt_enable();
>         worker->sleeping = 0;
>  }
>
> --
> 2.25.1
>