Re: [PATCH 1/2] bcache: ignore pending signals in bcache_device_init()

[Coly Li <colyli@suse.de>]

On 2020/3/3 1:19 上午, Jens Axboe wrote:
> On 3/2/20 10:16 AM, Coly Li wrote:
>> On 2020/3/2 9:49 下午, Oleg Nesterov wrote:
>>> On 03/02, Michal Hocko wrote:
>>>>
>>>> I cannot really comment on the bcache part because I am not familiar
>>>> with the code.
>>>
>>> same here...
>>>
>>>>> This patch calls flush_signals() in bcache_device_init() if there is
>>>>> pending signal for current process. It avoids bcache registration
>>>>> failure in system boot up time due to bcache udev rule timeout.
>>>>
>>>> this sounds like a wrong way to address the issue. Killing the udev
>>>> worker is a userspace policy and the kernel shouldn't simply ignore it.
>>>
>>> Agreed. If nothing else, if a userspace process has pending SIKILL then
>>> flush_signals() is very wrong.
>>>
>>>> Btw. Oleg, I have noticed quite a lot of flush_signals usage in the
>>>> drivers land and I have really hard time to understand their purpose.
>>>
>>> Heh. I bet most if not all users of flush_signals() are simply wrong.
>>>
>>>> What is the actual valid usage of this function?
>>>
>>> I thinks it should die... It was used by kthreads, but today
>>> signal_pending() == T is only possible if kthread does allow_signal(),
>>> and in this case it should probably use kernel_dequeue_signal().
>>>
>>>
>>> Say, io_sq_thread(). Why does it do
>>>
>>> 		if (signal_pending(current))
>>> 			flush_signals(current);
>>>
>>> afaics this kthread doesn't use allow_signal/allow_kernel_signal, this
>>> means that signal_pending() must be impossible even if this kthread sleeps
>>> in TASK_INTERRUPTIBLE state. Add Jens.
>>
>> Hi Oleg,
>>
>> Can I use disallow_signal() before the registration begins and use
>> allow_signal() after the registration done. Is this a proper way to
>> ignore the signal sent by udevd for timeout ?
>>
>> For me the above method seems to solve my problem too.
> 
> Really seems to me like you're going about this all wrong. The issue is
> that systemd is killing the startup, because it's taking too long. Don't
> try and work around that, ensure the timeout is appropriate.
> 

Copied. Then let me try how to make event_timeout works on my udevd. If
it works without other side effect, I will revert existing
flush_signals() patches.

> What if someone else tried to kill the startup? It'd be pretty
> frustrating that it was impossible, just because signals were blocked or
> flushed. The assumption that systemd is the ONLY task that would want to
> kill it is flawed.
> 

Indeed now the bcache registration can not be killed. I guess it is
because the mutex lock held during the metadata checking.
Sure I will look at how to extend udevd timeout value now, and ask for
help later.

Thanks.
-- 

Coly Li