Re: [Qemu-devel] [PULL 43/45] scsi: always call notifier on async cancellation

[Paolo Bonzini]

On 18/12/2015 01:57, Fam Zheng wrote:
> Oh hang on, in scsi_req_dequeue, if req->enqueued is already false, the
> matching scsi_req_unref is never called.

The matching unref for scsi_req_cancel_async's ref is in 
scsi_req_cancel_complete.  You're right that there is a leak if
we get to the second cancellation with req->aiocb, and we should
never get there with !req->aiocb.  So the patch is wrong, but
we should add some documentation instead of plainly reverting it:

diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c
index 00bddc9..378bf4d 100644
--- a/hw/scsi/scsi-bus.c
+++ b/hw/scsi/scsi-bus.c
@@ -1759,6 +1759,17 @@ void scsi_req_cancel_async(SCSIRequest *req, Notifier 
*notifier)
     if (notifier) {
         notifier_list_add(&req->cancel_notifiers, notifier);
     }
+    if (req->io_canceled) {
+        /* Canceling a second time after scsi_req_cancel_complete
+         * is a programming error, hence a blk_aio_cancel_async is
+         * pending; when it finishes, scsi_req_cancel_complete
+         * will be called and will call the notifier we just
+         * added.  Just wait for that.
+         */
+        assert(req->aiocb);
+        return;
+    }
+    /* Dropped in scsi_req_cancel_complete.  */
     scsi_req_ref(req);
     scsi_req_dequeue(req);
     req->io_canceled = true;
@@ -1775,6 +1784,8 @@ void scsi_req_cancel(SCSIRequest *req)
     if (!req->enqueued) {
         return;
     }
+    assert(!req->io_canceled);
+    /* Dropped in scsi_req_cancel_complete.  */
     scsi_req_ref(req);
     scsi_req_dequeue(req);
     req->io_canceled = true;

Does this look sane?

Thanks,

Paolo