回复: 回复: 回复: [PATCH] USB: usblp: Add device status detection in usblp_poll()

["Zhang, Qiang" <Qiang.Zhang@windriver.com>]

On Mon, Mar 01, 2021 at 08:52:58AM +0000, Zhang, Qiang wrote:
>
>
> ________________________________________
> 发件人: Greg KH <gregkh@linuxfoundation.org>
> 发送时间: 2021年3月1日 16:32
> 收件人: Zhang, Qiang
> 抄送: zaitcev@redhat.com; linux-usb@vger.kernel.org
> 主题: Re: 回复: [PATCH] USB: usblp: Add device status detection in usblp_poll()
>
> [Please note: This e-mail is from an EXTERNAL e-mail address]
>
> On Mon, Mar 01, 2021 at 08:26:22AM +0000, Zhang, Qiang wrote:
> >
> >
> > ________________________________________
> > 发件人: Greg KH <gregkh@linuxfoundation.org>
> > 发送时间: 2021年3月1日 16:00
> > 收件人: Zhang, Qiang
> > 抄送: zaitcev@redhat.com; linux-usb@vger.kernel.org
> > 主题: Re: [PATCH] USB: usblp: Add device status detection in usblp_poll()
> >
> > [Please note: This e-mail is from an EXTERNAL e-mail address]
> >
> > On Mon, Mar 01, 2021 at 12:03:00PM +0800, qiang.zhang@windriver.com wrote:
> > > From: Zqiang <qiang.zhang@windriver.com>
> > >
> > > If usblp device has been disconnected, direct return
> > > 'EPOLLHUP | EPOLLERR' in usblp_poll().
> > >
> > >Why?  What problem are you solving here?  Is this fixing an >older
> > >commit, if so, what one?
> > >
> >
> > There is not need fix older commit.
> > After the device is disconnected, if the userspace call >poll/select() function, will block and never be awakened .
> >
> >If the device is disconnected, how can the file be opened at >all?
> >Shouldn't it have been torn down already?
>
> If the device disconnect happen after the file be opend, and then we call poll/select() will always blocked.
> Also  before we operate the device, we need to judge the state of the >device. If it is disconnected, there is no need for subsequent >operation .
>
>But shouldn't the normal call to poll_wait() fail here?

poll_wait() is not fail,  its main operation is to call  add_wait_queue()

>
> >And now you are holding a lock across some poll_wait() calls, >is >that
> >going to cause problems?
>
> The 'usblp->present' is portect by 'usblp->mut',  when devices disconnect, will acquire 'usblp->mut', and set 'usblp->present'
> zero.
>
>But you are now holding a mutex across a very very long period of >time,
>what will that change?

maybe acquire mutex lock will take a long time,  but it's safe.

and also I find  similar usage in usb/class/usbtmc.c

thanks
Qiang
>
>thanks,
>
>greg k-h