Hi,
On Wed, 14 Apr 2021, xiaojun.zhao141@xxxxxxxxx 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
