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Date: Mon, 15 Apr 2024 17:04:52 +0200 (CEST)
From: Mikulas Patocka <mpatocka@redhat.com>
To: Li Lingfeng <lilingfeng3@huawei.com>
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Subject: Re: [PATCH 6.6] Revert "dm-crypt, dm-verity: disable tasklets"
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On Fri, 12 Apr 2024, Li Lingfeng wrote:

> Hi
> 
> I'm having difficulty understanding "Workqueues and ksoftirqd may be scheduled
> arbitrarily".
> This is my understanding:
> kcryptd_queue_crypt
> ?tasklet_schedule
> ? __tasklet_schedule
> ?? __tasklet_schedule_common
> ??? raise_softirq_irqoff
> ???? wakeup_softirqd
> ????? wake_up_process // ksoftirqd
> 
> run_ksoftirqd
> ?__do_softirq
> ? softirq_handle_begin
> ?? __local_bh_disable_ip // Turn off preemption
> <---------- [1] ---------->
> ? tasklet_action // h->action
> ?? tasklet_action_common
> ??? tasklet_trylock
> ???? kcryptd_crypt_tasklet // t->func(t->data)
> ???? ...
> ????? queue_work(cc->io_queue, &io->work)
> <---------- [2] ---------->
> ??? tasklet_unlock
> 
> // workqueue process
> kcryptd_io_bio_endio
> ?...
> ?// free tasklet_struct
> 
> Since preemption has been turned off at [1], I'm confused about how the CPU
> can be scheduled out to do work first at [2].
> Would you mind explaining it to me?
> 
> Thanks

Yes, you are right that scheduling is disabled when ksoftirqd processes a 
softirq task.

But the upstream kernel switched to bh workqueues anyway, so there is no 
need to submit a different solution to the stable kernels.

Mikulas
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