diff mbox series

USB: usblp: Add device status detection in usblp_poll()

Message ID 20210301040300.20834-1-qiang.zhang@windriver.com
State New
Headers show
Series USB: usblp: Add device status detection in usblp_poll() | expand

Commit Message

Zhang, Qiang March 1, 2021, 4:03 a.m. UTC 
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(+)

Comments

Greg KH March 2, 2021, 7:04 a.m. UTC | #1 
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
Zhang, Qiang March 2, 2021, 7:41 a.m. UTC | #2 
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
Pete Zaitcev March 3, 2021, 5:12 a.m. UTC | #3 
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;
 }
Greg KH March 3, 2021, 6:56 a.m. UTC | #4 
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
Pete Zaitcev March 3, 2021, 3:57 p.m. UTC | #5 
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
Alan Stern March 3, 2021, 5:13 p.m. UTC | #6 
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;

>  }

>
diff mbox series

Patch

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;
 }