From: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Date: Wed, 8 Sep 2010 22:10:42 +0000 (-0400)
Subject: xenbus: avoid zero returns from read()
X-Git-Tag: firefly_0821_release~7613^2~2634^2~1
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=7808121b9a1e44ef12fecd49fa6c268f27a150fc;p=firefly-linux-kernel-4.4.55.git

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>
---

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);