>________________________________________
>发件人: Pete Zaitcev <zaitcev@xxxxxxxxxx>
>发送时间: 2021年3月3日 13:12
>收件人: Zhang, Qiang
>抄送: Greg KH; linux-usb@xxxxxxxxxxxxxxx; zaitcev@xxxxxxxxxx
>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 Tue, 2 Mar 2021 07:41:07 +0000
>"Zhang, Qiang" <Qiang.Zhang@xxxxxxxxxxxxx> wrote:
> and also I find similar usage in usb/class/usbtmc.c
>Seems like a bug indeed, but I don't like the example in usbtmc.c.
>Please let me know if the following is acceptable:
I agree with this change .
Thanks
Qiang
>
>commit 83591ac63bc666a44f250b43af6c0f5a1e001841
>Author: Pete Zaitcev <zaitcev@xxxxxxxxxxxxxxxxx>
>Date: Tue Mar 2 23:00:28 2021 -0600
>
> usblp: fix a hang in poll() if disconnected
>
> Apparently an application that opens a device and calls select()
> on it, will hang if the decice is disconnected. It's a little
> surprising that we had this bug for 15 years, but apparently
> nobody ever uses select() with a printer: only write() and read(),
> and those work fine. Well, you can also select() with a timeout.
>
> The fix is modeled after devio.c. A few drivers check the
> condition first, then do not add the wait queue in case the
> device is disconnected. We doubt that's completely race-free.
> So, this patch adds the process first, then locks properly
> and checks for the disconnect.
>
> Reported-by: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
> Signed-off-by: Pete Zaitcev <zaitcev@xxxxxxxxxx>
>
>diff --git a/drivers/usb/class/usblp.c b/drivers/usb/class/usblp.c
>index fd87405adbed..5733a0067f5b 100644
>--- a/drivers/usb/class/usblp.c
>+++ b/drivers/usb/class/usblp.c
>@@ -494,16 +494,24 @@ static int usblp_release(struct inode *inode, >struct file *file)
>/* No kernel lock - fine */
> static __poll_t usblp_poll(struct file *file, struct poll_table_struct >*wait)
> {
>- __poll_t ret;
>+ struct usblp *usblp = file->private_data;
>+ __poll_t ret = 0;
> unsigned long flags;
>
>- struct usblp *usblp = file->private_data;
> /* Should we check file->f_mode & FMODE_WRITE before >poll_wait()? */
> poll_wait(file, &usblp->rwait, wait);
> poll_wait(file, &usblp->wwait, wait);
>+
>+ mutex_lock(&usblp->mut);
>+ if (!usblp->present)
>+ ret != EPOLLHUP;
>+ mutex_unlock(&usblp->mut);
>+
> spin_lock_irqsave(&usblp->lock, flags);
>- ret = ((usblp->bidir && usblp->rcomplete) ? EPOLLIN | >EPOLLRDNORM : 0) |
>- ((usblp->no_paper || usblp->wcomplete) ? EPOLLOUT | >EPOLLWRNORM : 0);
>+ if (usblp->bidir && usblp->rcomplete)
>+ ret |= EPOLLIN | EPOLLRDNORM;
>+ if (usblp->no_paper || usblp->wcomplete)
>+ ret |= EPOLLOUT | EPOLLWRNORM;
> spin_unlock_irqrestore(&usblp->lock, flags);
> return ret;
> }
