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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

 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

>> 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@xxxxxxxxxxxxx]
>> 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@xxxxxxxxxxxxx>
>>
>> ---
>> 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);
>


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