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>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/drivers/xen/xenfs/xenbus.c b/drivers/xen/xenfs/xenbus.c
index c4c7db8363e7..55791dd1105f 100644
--- a/drivers/xen/xenfs/xenbus.c
+++ b/drivers/xen/xenfs/xenbus.c
@@ -120,6 +120,7 @@ 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)
@@ -158,6 +159,8 @@ static ssize_t xenbus_file_read(struct file *filp,
                                         struct read_buffer, list);
                 }
         }
+        if (i == 0)
+                goto again;
 
 out:
         mutex_unlock(&u->reply_mutex);