linux-kernel - Re: [syzbot] possible deadlock in worker

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Date:   Tue, 15 Feb 2022 21:48:58 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Haakon Bugge <haakon.bugge@...cle.com>
Cc:     Tejun Heo <tj@...nel.org>, Bart Van Assche <bvanassche@....org>,
        syzbot <syzbot+831661966588c802aae9@...kaller.appspotmail.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        LKML <linux-kernel@...r.kernel.org>,
        OFED mailing list <linux-rdma@...r.kernel.org>,
        "syzkaller-bugs@...glegroups.com" <syzkaller-bugs@...glegroups.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [syzbot] possible deadlock in worker_thread

On 2022/02/15 19:43, Haakon Bugge wrote:
>> @@ -6070,6 +6087,13 @@ void __init workqueue_init_early(void)
>> 	       !system_unbound_wq || !system_freezable_wq ||
>> 	       !system_power_efficient_wq ||
>> 	       !system_freezable_power_efficient_wq);
>> +	system_wq->flags |= __WQ_SYSTEM_WIDE;
>> +	system_highpri_wq->flags |= __WQ_SYSTEM_WIDE;
>> +	system_long_wq->flags |= __WQ_SYSTEM_WIDE;
>> +	system_unbound_wq->flags |= __WQ_SYSTEM_WIDE;
>> +	system_freezable_wq->flags |= __WQ_SYSTEM_WIDE;
>> +	system_power_efficient_wq->flags |= __WQ_SYSTEM_WIDE;
>> +	system_freezable_power_efficient_wq->flags |= __WQ_SYSTEM_WIDE;
> 
> Better to OR this in, in the alloc_workqueue() call? Perceive the notion of an opaque object?
> 

I do not want to do like

-	system_wq = alloc_workqueue("events", 0, 0);
+	system_wq = alloc_workqueue("events", __WQ_SYSTEM_WIDE, 0);

because the intent of this change is to ask developers to create their own WQs.
If I pass __WQ_SYSTEM_WIDE to alloc_workqueue(), developers might by error create like

	srp_tl_err_wq = alloc_workqueue("srp_tl_err_wq", __WQ_SYSTEM_WIDE, 0);

because of

	system_wq = alloc_workqueue("events", __WQ_SYSTEM_WIDE, 0);

line. The __WQ_SYSTEM_WIDE is absolutely meant to be applied to only 'system_wq',
'system_highpri_wq', 'system_long_wq', 'system_unbound_wq', 'system_freezable_wq',
'system_power_efficient_wq' and 'system_freezable_power_efficient_wq' WQs, in order
to avoid calling flush_workqueue() on these system-wide WQs.

I wish I could define __WQ_SYSTEM_WIDE inside kernel/workqueue_internal.h, but
it seems that kernel/workqueue_internal.h does not define internal flags.