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

From: Coly Li <colyli@suse.de>
To: Jens Axboe <axboe@kernel.dk>
Cc: Oleg Nesterov <oleg@redhat.com>, Michal Hocko <mhocko@kernel.org>,
	linux-bcache@vger.kernel.org, linux-block@vger.kernel.org,
	hare@suse.de, mkoutny@suse.com
Subject: Re: [PATCH 1/2] bcache: ignore pending signals in bcache_device_init()
Date: Tue, 3 Mar 2020 09:08:08 +0800	[thread overview]
Message-ID: <c763699f-108a-f323-dbf1-235cf27b21e4@suse.de> (raw)
In-Reply-To: <64c2a8e9-1dc9-f81d-1c11-d9c4e7e0fd2b@kernel.dk>

On 2020/3/3 4:33 上午, Jens Axboe wrote:
> On 3/2/20 10:32 AM, Coly Li wrote:
>> 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.
> 
> Thanks, this one, right?
> 
> commit 0b96da639a4874311e9b5156405f69ef9fc3bef8
> Author: Coly Li <colyli@suse.de>
> Date:   Thu Feb 13 22:12:05 2020 +0800
> 
>     bcache: ignore pending signals when creating gc and allocator thread
> 
> because that definitely needs to be reverted.
> 

Yes, please revert this commit. Thank you.

-- 

Coly Li