xen/mce: add .poll method for mcelog device driver

If a driver leaves its poll method NULL, the device is assumed to
be both readable and writable without blocking.

This patch add .poll method to xen mcelog device driver, so that
when mcelog use system calls like ppoll or select, it would be
blocked when no data available, and avoid spinning at CPU.
Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/mcelog.c

@@ -41,6 +41,8 @@
 #include <linux/miscdevice.h>
 #include <linux/uaccess.h>
 #include <linux/capability.h>
+#include <linux/poll.h>
+#include <linux/sched.h>
 
 #include <xen/interface/xen.h>
 #include <xen/events.h>
@@ -67,6 +69,8 @@ static DEFINE_SPINLOCK(xen_mce_chrdev_state_lock);
 static int xen_mce_chrdev_open_count;	/* #times opened */
 static int xen_mce_chrdev_open_exclu;	/* already open exclusive? */
 
+static DECLARE_WAIT_QUEUE_HEAD(xen_mce_chrdev_wait);
+
 static int xen_mce_chrdev_open(struct inode *inode, struct file *file)
 {
 	spin_lock(&xen_mce_chrdev_state_lock);
@@ -135,6 +139,16 @@ static ssize_t xen_mce_chrdev_read(struct file *filp, char __user *ubuf,
 	return err ? err : buf - ubuf;
 }
 
+static unsigned int xen_mce_chrdev_poll(struct file *file, poll_table *wait)
+{
+	poll_wait(file, &xen_mce_chrdev_wait, wait);
+
+	if (xen_mcelog.next)
+		return POLLIN | POLLRDNORM;
+
+	return 0;
+}
+
 static long xen_mce_chrdev_ioctl(struct file *f, unsigned int cmd,
 				unsigned long arg)
 {
@@ -166,6 +180,7 @@ static const struct file_operations xen_mce_chrdev_ops = {
 	.open			= xen_mce_chrdev_open,
 	.release		= xen_mce_chrdev_release,
 	.read			= xen_mce_chrdev_read,
+	.poll			= xen_mce_chrdev_poll,
 	.unlocked_ioctl		= xen_mce_chrdev_ioctl,
 	.llseek			= no_llseek,
 };
@@ -329,6 +344,9 @@ static void xen_mce_work_fn(struct work_struct *work)
 		pr_err(XEN_MCELOG
 		       "Failed to handle nonurgent mc_info queue.\n");
 
+	/* wake processes polling /dev/mcelog */
+	wake_up_interruptible(&xen_mce_chrdev_wait);
+
 	mutex_unlock(&mcelog_lock);
 }
 static DECLARE_WORK(xen_mce_work, xen_mce_work_fn);