Re: [PATCH] xenbus: avoid zero returns from read()

[Daniel De Graaf <dgdegra@tycho.nsa.gov>]

On 09/09/2010 11:25 AM, Jeremy Fitzhardinge wrote:
>  On 09/10/2010 12:40 AM, Daniel De Graaf wrote:
>> On 09/09/2010 05:27 AM, Jun Zhu (Intern) wrote:
>>> Is it related to the following patch? The following patch fixes the problem of queue deletion.
>> Yes, this would have caused a zero return due to an empty item being left
>> in the queue. It's likely my patch is not needed with this one applied; we
>> are already careful to avoid adding zero-length elements to the queue,
>> which was my original idea for the cause.
> 
> No, that's a separate issue.  This patch fixes the case where the
> usermode buffer is near a page boundary so the copy to usermode is
> truncated.
> 
> Anyway, I've applied both.
> 
>     J

One or both of these patches are needed in the 2.6.37 kernel; I have
observed a zero return from a read() on xenbus on 2.6.37-rc8 that is
fixed by this patch.

>>> diff --git a/drivers/xen/xenfs/xenbus.c b/drivers/xen/xenfs/xenbus.c
>>> index 64b3be4..763e90d 100644
>>> --- a/drivers/xen/xenfs/xenbus.c
>>> +++ b/drivers/xen/xenfs/xenbus.c
>>> @@ -143,7 +143,7 @@ static ssize_t xenbus_file_read(struct file *filp,
>>>                 i += sz - ret;
>>>                 rb->cons += sz - ret;
>>>  
>>> -               if (ret != sz) {
>>> +               if (ret != 0) {
>>>                         if (i == 0)
>>>                                 i = -EFAULT;
>>>                         goto out;
>>>
>>> Jun Zhu
>>> Citrix Systems UK
>>> ________________________________________
>>> From: Daniel De Graaf [dgdegra@tycho.nsa.gov]
>>> Sent: 08 September 2010 18:10
>>> To: Jeremy Fitzhardinge
>>> Cc: xen-devel; Jun Zhu (Intern)
>>> Subject: [PATCH] xenbus: avoid zero returns from read()
>>>
>>> It is possible to get a zero return from read() in instances where the
>>> queue is not empty but has no elements with data to deliver to the user.
>>> Since a zero return from read is an error indicator, resume waiting or
>>> return -EAGAIN (for a nonblocking fd) in this case.
>>>
>>> Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
>>>
>>> ---
>>> diff --git a/drivers/xen/xenfs/xenbus.c b/drivers/xen/xenfs/xenbus.c
>>> index 88c87c9..0ddef43 100644
>>> --- a/drivers/xen/xenfs/xenbus.c
>>> +++ b/drivers/xen/xenfs/xenbus.c
>>> @@ -121,6 +121,7 @@ static ssize_t xenbus_file_read(struct file *filp,
>>>         int ret;
>>>
>>>         mutex_lock(&u->reply_mutex);
>>> +again:
>>>         while (list_empty(&u->read_buffers)) {
>>>                 mutex_unlock(&u->reply_mutex);
>>>                 if (filp->f_flags & O_NONBLOCK)
>>> @@ -159,6 +160,8 @@ static ssize_t xenbus_file_read(struct file *filp,
>>>                                         struct read_buffer, list);
>>>                 }
>>>         }
>>> +       if (i == 0)
>>> +               goto again;
>>>
>>>  out:
>>>         mutex_unlock(&u->reply_mutex);
>>
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel


-- 
Daniel De Graaf
National Security Agency