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

Commit Message

Xiao Guangrong Aug. 21, 2018, 8:10 a.m. UTC

From: Xiao Guangrong <[e-mail]>

Try to hold src_page_req_mutex only if the queue is not
empty

Reviewed-by: Dr. David Alan Gilbert <[e-mail]>
Reviewed-by: Peter Xu <[e-mail]>
Signed-off-by: Xiao Guangrong <[e-mail]>
---
 include/qemu/queue.h | 1 +
 migration/ram.c      | 4 ++++
 2 files changed, 5 insertions(+)

Comments

Juan Quintela Aug. 22, 2018, 10:24 a.m. UTC | #1

[e-mail] wrote:
> From: Xiao Guangrong <[e-mail]>
>
> Try to hold src_page_req_mutex only if the queue is not
> empty
>
> Reviewed-by: Dr. David Alan Gilbert <[e-mail]>
> Reviewed-by: Peter Xu <[e-mail]>
> Signed-off-by: Xiao Guangrong <[e-mail]>

Reviewed-by: Juan Quintela <[e-mail]>

Patch

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 =