Re: [PATCH -next V3] ubi: fix race condition between ctrl_cdev_ioctl and ubi_cdev_ioctl

[Zhihao Cheng]

在 2021/12/24 5:06, Richard Weinberger 写道:
> ----- Ursprüngliche Mail -----


> > > ubi_get_device is used to prevent devices from being deleted during
> > > sysfs execution. However, now kernfs ensures that devices will not
> > > be deleted before all reference counting are released.
> > > The key process is shown in the following stack.
>
> ubi_get_device() in dev_attribute_show() is used to detect whether
> the ubi device got detached while the sysfs file is open.
>
> Hmm. I thought for sysfs this is not the case since sysfs does not implement
> a release() method. So kernfs_drain_open_files() will return early.
> But there is a good chance that I don't got all kernfs/sysfs details.

kernfs_drain() will wait 'root->deactivate_waitq' if 
atomic_read(&kn->active) not equals to KN_DEACTIVATED_BIAS.

The UBI seq_show callback is invoked with avtive cnt taken:
vfs_read
  kernfs_fop_read_iter
    seq_read_iter
      m->op->start (kernfs_seq_start)   // kernfs_get_active(of->kn)
      kernfs_seq_show
        dev_attribute_show [ubi]
      m->op->stop (kernfs_seq_stop)     // kernfs_put_active(of->kn)

The kernfs_drain() is stuck at wait_event() until sysfs reading 
finished, in my local test.