[PATCH v4 06/10] migration: hold the lock only if it is really needed

[guangrong.xiao@xxxxxxxxx]

From: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxx>

Try to hold src_page_req_mutex only if the queue is not
empty

Reviewed-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx>
Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxx>
---
 include/qemu/queue.h | 1 +
 migration/ram.c      | 4 ++++
 2 files changed, 5 insertions(+)

diff --git a/include/qemu/queue.h b/include/qemu/queue.h
index 59fd1203a1..ac418efc43 100644
--- a/include/qemu/queue.h
+++ b/include/qemu/queue.h
@@ -341,6 +341,7 @@ struct {                                                                \
 /*
  * Simple queue access methods.
  */
+#define QSIMPLEQ_EMPTY_ATOMIC(head) (atomic_read(&((head)->sqh_first)) == NULL)
 #define QSIMPLEQ_EMPTY(head)        ((head)->sqh_first == NULL)
 #define QSIMPLEQ_FIRST(head)        ((head)->sqh_first)
 #define QSIMPLEQ_NEXT(elm, field)   ((elm)->field.sqe_next)
diff --git a/migration/ram.c b/migration/ram.c
index d804d01aae..99ecf9b315 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -2026,6 +2026,10 @@ static RAMBlock *unqueue_page(RAMState *rs, ram_addr_t *offset)
 {
     RAMBlock *block = NULL;
 
+    if (QSIMPLEQ_EMPTY_ATOMIC(&rs->src_page_requests)) {
+        return NULL;
+    }
+
     qemu_mutex_lock(&rs->src_page_req_mutex);
     if (!QSIMPLEQ_EMPTY(&rs->src_page_requests)) {
         struct RAMSrcPageRequest *entry =
-- 
2.14.4