async: optimize aio_bh_poll

Avoid entering the slow path of qemu_lockcnt_dec_and_lock if
no bottom half has to be deleted.
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Message-id: 20170112180800.21085-11-pbonzini@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

async.c

@@ -92,6 +92,7 @@ int aio_bh_poll(AioContext *ctx)
 {
     QEMUBH *bh, **bhp, *next;
     int ret;
+    bool deleted = false;
 
     qemu_lockcnt_inc(&ctx->list_lock);
 
@@ -112,9 +113,17 @@ int aio_bh_poll(AioContext *ctx)
             bh->idle = 0;
             aio_bh_call(bh);
         }
+        if (bh->deleted) {
+            deleted = true;
+        }
     }
 
     /* remove deleted bhs */
+    if (!deleted) {
+        qemu_lockcnt_dec(&ctx->list_lock);
+        return ret;
+    }
+
     if (qemu_lockcnt_dec_and_lock(&ctx->list_lock)) {
         bhp = &ctx->first_bh;
         while (*bhp) {
@@ -128,7 +137,6 @@ int aio_bh_poll(AioContext *ctx)
         }
         qemu_lockcnt_unlock(&ctx->list_lock);
     }
-
     return ret;
 }