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Subject: Re: the qemu-nbd process automatically exit with the commit 43347d56c
 'livepatch: send a fake signal to all blocking tasks'
To:     xiaojun.zhao141@gmail.com, Miroslav Benes <mbenes@suse.cz>
Cc:     linux-kernel@vger.kernel.org, live-patching@vger.kernel.org
References: <20210414115548.0cdb529b@slime>
 <alpine.LSU.2.21.2104141320060.6604@pobox.suse.cz>
 <20210414232119.13b126fa@slime>
From:   Josef Bacik <josef@toxicpanda.com>
Message-ID: <f7698105-23a4-4558-7b65-9116e8587848@toxicpanda.com>
Date:   Wed, 14 Apr 2021 13:21:37 -0400
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On 4/14/21 11:21 AM, xiaojun.zhao141@gmail.com wrote:
> On Wed, 14 Apr 2021 13:27:43 +0200 (CEST)
> Miroslav Benes <mbenes@suse.cz> wrote:
> 
>> Hi,
>>
>> On Wed, 14 Apr 2021, xiaojun.zhao141@gmail.com wrote:
>>
>>> I found the qemu-nbd process(started with qemu-nbd -t -c /dev/nbd0
>>> nbd.qcow2) will automatically exit when I patched for functions of
>>> the nbd with livepatch.
>>>
>>> The nbd relative source:
>>> static int nbd_start_device_ioctl(struct nbd_device *nbd, struct
>>> block_device *bdev)
>>> { struct nbd_config *config =
>>> nbd->config; int
>>> ret;
>>>          ret =
>>> nbd_start_device(nbd); if
>>> (ret) return
>>> ret;
>>>          if
>>> (max_part) bdev->bd_invalidated =
>>> 1;
>>> mutex_unlock(&nbd->config_lock); ret =
>>> wait_event_interruptible(config->recv_wq,
>>> atomic_read(&config->recv_threads) == 0); if
>>> (ret)
>>> sock_shutdown(nbd);
>>> flush_workqueue(nbd->recv_workq);
>>>          mutex_lock(&nbd->config_lock);
>>>          nbd_bdev_reset(bdev);
>>>          /* user requested, ignore socket errors
>>> */ if (test_bit(NBD_RT_DISCONNECT_REQUESTED,
>>> &config->runtime_flags)) ret =
>>> 0; if (test_bit(NBD_RT_TIMEDOUT,
>>> &config->runtime_flags)) ret =
>>> -ETIMEDOUT; return
>>> ret; }
>>
>> So my understanding is that ndb spawns a number
>> (config->recv_threads) of workqueue jobs and then waits for them to
>> finish. It waits interruptedly. Now, any signal would make
>> wait_event_interruptible() to return -ERESTARTSYS. Livepatch fake
>> signal is no exception there. The error is then propagated back to
>> the userspace. Unless a user requested a disconnection or there is
>> timeout set. How does the userspace then reacts to it? Is
>> _interruptible there because the userspace sends a signal in case of
>> NBD_RT_DISCONNECT_REQUESTED set? How does the userspace handles
>> ordinary signals? This all sounds a bit strange, but I may be missing
>> something easily.
>>
>>> When the nbd waits for atomic_read(&config->recv_threads) == 0, the
>>> klp will send a fake signal to it then the qemu-nbd process exits.
>>> And the signal of sysfs to control this action was removed in the
>>> commit 10b3d52790e 'livepatch: Remove signal sysfs attribute'. Are
>>> there other ways to control this action? How?
>>
>> No, there is no way currently. We send a fake signal automatically.
>>
>> Regards
>> Miroslav
> It occurs IO error of the nbd device when I use livepatch of the
> nbd, and I guess that any livepatch on other kernel source maybe cause
> the IO error. Well, now I decide to workaround for this problem by
> adding a livepatch for the klp to disable a automatic fake signal.
> 

Would wait_event_killable() fix this problem?  I'm not sure any client 
implementations depend on being able to send other signals to the client 
process, so it should be safe from that standpoint.  Not sure if the livepatch 
thing would still get an error at that point tho.  Thanks,

Josef