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From:   xiaojun.zhao141@gmail.com
Date:   Wed, 14 Apr 2021 11:55:48 +0800
To:     mbenes@suse.cz
Cc:     josef@toxicpanda.com, linux-kernel@vger.kernel.org
Subject: the qemu-nbd process automatically exit with the commit 43347d56c
 'livepatch: send a fake signal to all blocking tasks'
Message-ID: <20210414115548.0cdb529b@slime>
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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;                                                             
}

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?

Thanks very much.