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

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

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.

> 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);


-- 
Daniel De Graaf
National Security Agency

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