[Qemu-devel] [PATCH 1/3] QEMUBH: introduce canceled member for bh

[Liu Ping Fan]

When dismissing the bh scheduling(delete/cancel/schedule), we need to
reset bh->scheduled to zero and release the refcnt of object which is
referred by bh(will introduced by next patch).
Currently, the bh->scheduled will be reset to zero by many writers,
so atomic ops should be involved in, which results in expensive memory
barrier. With this patch, bh->scheduled is only reset by aio_bh_poll().

Signed-off-by: Liu Ping Fan <pingf...@linux.vnet.ibm.com>
---
 async.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/async.c b/async.c
index e73b93c..bea3d7e 100644
--- a/async.c
+++ b/async.c
@@ -38,6 +38,7 @@ struct QEMUBH {
     bool scheduled;
     bool idle;
     bool deleted;
+    bool canceled;
 };
 
 QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void *opaque)
@@ -69,8 +70,15 @@ int aio_bh_poll(AioContext *ctx)
         /* Make sure that fetching bh happens before accessing its members */
         smp_read_barrier_depends();
         next = bh->next;
-        if (!bh->deleted && bh->scheduled) {
+        if (bh->scheduled) {
             bh->scheduled = 0;
+            if (unlikely(bh->deleted)) {
+                continue;
+            }
+            if (unlikely(bh->canceled)) {
+                bh->canceled = 0;
+                continue;
+            }
             /* Paired with write barrier in bh schedule to ensure reading for
              * idle & callbacks coming after bh's scheduling.
              */
@@ -133,7 +141,7 @@ void qemu_bh_schedule(QEMUBH *bh)
  */
 void qemu_bh_cancel(QEMUBH *bh)
 {
-    bh->scheduled = 0;
+    bh->canceled = 1;
 }
 
 /* This func is async.The bottom half will do the delete action at the finial
@@ -141,7 +149,6 @@ void qemu_bh_cancel(QEMUBH *bh)
  */
 void qemu_bh_delete(QEMUBH *bh)
 {
-    bh->scheduled = 0;
     bh->deleted = 1;
 }
 
@@ -152,7 +159,7 @@ aio_ctx_prepare(GSource *source, gint    *timeout)
     QEMUBH *bh;
 
     for (bh = ctx->first_bh; bh; bh = bh->next) {
-        if (!bh->deleted && bh->scheduled) {
+        if (!bh->deleted && !bh->canceled && bh->scheduled) {
             if (bh->idle) {
                 /* idle bottom halves will be polled at least
                  * every 10ms */
@@ -176,7 +183,7 @@ aio_ctx_check(GSource *source)
     QEMUBH *bh;
 
     for (bh = ctx->first_bh; bh; bh = bh->next) {
-        if (!bh->deleted && bh->scheduled) {
+        if (!bh->deleted && !bh->canceled && bh->scheduled) {
             return true;
        }
     }
-- 
1.8.1.4