[PATCH] xen: release lock occasionally during ballooning

[Juergen Gross <jgross@suse.com>]

When dom0 is being ballooned balloon_process() will hold the balloon
mutex until it is finished. This will block e.g. creation of new
domains as the device backends for the new domain need some
autoballooned pages for the ring buffers.

Avoid this by releasing the balloon mutex from time to time during
ballooning. Add a state variable to indicate one balloon_process()
is active to avoid multiple balloon processes fighting for the mutex.

Instead of open coding it, just use cond_resched().

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 drivers/xen/balloon.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index fd93369..e6d9eee 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -481,9 +481,16 @@ static void balloon_process(struct work_struct *work)
 {
 	enum bp_state state = BP_DONE;
 	long credit;
+	static bool active;
 
 	mutex_lock(&balloon_mutex);
 
+	if (active) {
+		mutex_unlock(&balloon_mutex);
+		return;
+	}
+	active = true;
+
 	do {
 		credit = current_credit();
 
@@ -499,12 +506,16 @@ static void balloon_process(struct work_struct *work)
 
 		state = update_schedule(state);
 
-#ifndef CONFIG_PREEMPT
-		if (need_resched())
-			schedule();
-#endif
+		mutex_unlock(&balloon_mutex);
+
+		cond_resched();
+
+		mutex_lock(&balloon_mutex);
+
 	} while (credit && state == BP_DONE);
 
+	active = false;
+
 	/* Schedule more work if there is some still to be done. */
 	if (state == BP_EAGAIN)
 		schedule_delayed_work(&balloon_worker, balloon_stats.schedule_delay * HZ);
-- 
2.1.4