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

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

[qiang.zhang]

From: Zqiang <qiang.zhang@windriver.com>

If usblp device has been disconnected, direct return
'EPOLLHUP | EPOLLERR' in usblp_poll().

Signed-off-by: Zqiang <qiang.zhang@windriver.com>
---
 drivers/usb/class/usblp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/usb/class/usblp.c b/drivers/usb/class/usblp.c
index c9f6e9758288..40095c8826d2 100644
--- a/drivers/usb/class/usblp.c
+++ b/drivers/usb/class/usblp.c
@@ -498,6 +498,12 @@ static __poll_t usblp_poll(struct file *file, struct poll_table_struct *wait)
 	unsigned long flags;
 
 	struct usblp *usblp = file->private_data;
+	mutex_lock(&usblp->mut);
+	if (!usblp->present) {
+		ret = EPOLLHUP | EPOLLERR;
+		goto no_poll;
+	}
+
 	/* Should we check file->f_mode & FMODE_WRITE before poll_wait()? */
 	poll_wait(file, &usblp->rwait, wait);
 	poll_wait(file, &usblp->wwait, wait);
@@ -505,6 +511,8 @@ static __poll_t usblp_poll(struct file *file, struct poll_table_struct *wait)
 	ret = ((usblp->bidir && usblp->rcomplete) ? EPOLLIN  | EPOLLRDNORM : 0) |
 	   ((usblp->no_paper || usblp->wcomplete) ? EPOLLOUT | EPOLLWRNORM : 0);
 	spin_unlock_irqrestore(&usblp->lock, flags);
+no_poll:
+	mutex_unlock(&usblp->mut);
 	return ret;
 }
 
-- 
2.25.1

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

[Greg KH]

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?

Please provide a lot more information here, this is not very
descriptive.

thanks,

greg k-h

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

[Zhang, Qiang]

________________________________________
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 .

Thanks
Qiang 

>Please provide a lot more information here, this is not very
>descriptive.
>
>thanks,
>
>greg k-h

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

[Greg KH]

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?

And now you are holding a lock across some poll_wait() calls, is that
going to cause problems?

thanks,

greg k-h

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

[Zhang, Qiang]

________________________________________
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 .

>
>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. 

Thanks
Qiang

>
>thanks,
>
>greg k-h

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

[Greg KH]

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?

> >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?

thanks,

greg k-h

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

[Zhang, Qiang]

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

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

[Pete Zaitcev]

On Tue, 2 Mar 2021 07:41:07 +0000
"Zhang, Qiang" <Qiang.Zhang@windriver.com> 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:

commit 83591ac63bc666a44f250b43af6c0f5a1e001841
Author: Pete Zaitcev <zaitcev@kotori.zaitcev.us>
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@windriver.com>
    Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>

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;
 }

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

[Zhang, Qiang]

>________________________________________
>发件人: Pete Zaitcev <zaitcev@redhat.com>
>发送时间: 2021年3月3日 13:12
>收件人: Zhang, Qiang
>抄送: Greg KH; linux-usb@vger.kernel.org; zaitcev@redhat.com
>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@windriver.com> 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@kotori.zaitcev.us>
>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@windriver.com>
>    Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
>
>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;
> }

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

[Greg KH]

On Wed, Mar 03, 2021 at 06:12:47AM +0000, Zhang, Qiang wrote:
> 
> 
> >________________________________________
> >发件人: Pete Zaitcev <zaitcev@redhat.com>
> >发送时间: 2021年3月3日 13:12
> >收件人: Zhang, Qiang
> >抄送: Greg KH; linux-usb@vger.kernel.org; zaitcev@redhat.com
> >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@windriver.com> 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 .

Great, can I get a "Reviewed-by:" line from you then?

thanks,

greg k-h

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

[Zhang, Qiang]

________________________________________
发件人: Zhang, Qiang <Qiang.Zhang@windriver.com>
发送时间: 2021年3月3日 14:12
收件人: Pete Zaitcev
抄送: Greg KH; linux-usb@vger.kernel.org
主题: 回复: [PATCH] USB: usblp: Add device status detection in usblp_poll()



>________________________________________
>发件人: Pete Zaitcev <zaitcev@redhat.com>
>发送时间: 2021年3月3日 13:12
>收件人: Zhang, Qiang
>抄送: Greg KH; linux-usb@vger.kernel.org; zaitcev@redhat.com
>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@windriver.com> 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@kotori.zaitcev.us>
>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@windriver.com>
>    Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
>
>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;


Should it be  ret |= EPOLLHUP;  
Reviewed-by: Zqiang <qiang.zhang@windriver.com>

Thanks
Qiang

>+       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;
> }

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

[Pete Zaitcev]

On Wed, 3 Mar 2021 07:15:08 +0000
"Zhang, Qiang" <Qiang.Zhang@windriver.com> wrote:

> >+       if (!usblp->present)
> >+               ret != EPOLLHUP;

> Should it be  ret |= EPOLLHUP;  
> Reviewed-by: Zqiang <qiang.zhang@windriver.com>

Ouch. I'll fix that, do some (sadly minimal) tests, and submit.

-- Pete

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

[Alan Stern]

On Tue, Mar 02, 2021 at 11:12:54PM -0600, Pete Zaitcev wrote:
> On Tue, 2 Mar 2021 07:41:07 +0000
> "Zhang, Qiang" <Qiang.Zhang@windriver.com> 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:
> 
> commit 83591ac63bc666a44f250b43af6c0f5a1e001841
> Author: Pete Zaitcev <zaitcev@kotori.zaitcev.us>
> 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@windriver.com>
>     Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
> 
> 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;

Typo: ! instead of |.  You have to look closely to see the difference.

alan Stern

> +	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;
>  }
>