Re: [Qemu-devel] [PATCH 0/4] Tweaks around virtio-blk start/stop

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/03/2016 13:39, Christian Borntraeger wrote:
> As an interesting side note, I updated my system from F20 to F23 some days ago
> (after the initial report). While To Bo is still on a F20 system. I was not able
> to reproduce the original crash on f23. but going back to F20 made this
> problem re-appear.
>  
>   Stack trace of thread 26429:
>                 #0  0x00000000802008aa tracked_request_begin (qemu-system-s390x)
>                 #1  0x0000000080203f3c bdrv_co_do_preadv (qemu-system-s390x)
>                 #2  0x000000008020567c bdrv_co_do_readv (qemu-system-s390x)
>                 #3  0x000000008025d0f4 coroutine_trampoline (qemu-system-s390x)
>                 #4  0x000003ff943d150a __makecontext_ret (libc.so.6)
> 
> this is with patch 2-4 plus the removal of virtio_queue_host_notifier_read.
> 
> Without removing virtio_queue_host_notifier_read, I get the same mutex lockup (as expected).
> 
> Maybe we have two independent issues here and this is some old bug in glibc or
> whatever?

I'm happy to try and reproduce on x86 if you give me some instruction
(RHEL7 should be close enough to Fedora 20).

Can you add an assert in virtio_blk_handle_output to catch reentrancy, like

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index a7ec572..96ea896 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -591,6 +591,8 @@ static void virtio_blk_handle_output(VirtIODevice
*vdev, VirtQueue *vq)
         return;
     }

+    int x = atomic_fetch_inc(&s->test);
+    assert(x == 0);
     blk_io_plug(s->blk);

     while ((req = virtio_blk_get_request(s))) {
@@ -602,6 +604,7 @@ static void virtio_blk_handle_output(VirtIODevice
*vdev, VirtQueue *vq)
     }

     blk_io_unplug(s->blk);
+    atomic_dec(&s->test);
 }

 static void virtio_blk_dma_restart_bh(void *opaque)
diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
index ae84d92..6472503 100644
--- a/include/hw/virtio/virtio-blk.h
+++ b/include/hw/virtio/virtio-blk.h
@@ -48,6 +48,7 @@ typedef struct VirtIOBlock {
     BlockBackend *blk;
     VirtQueue *vq;
     void *rq;
+    int test;
     QEMUBH *bh;
     VirtIOBlkConf conf;
     unsigned short sector_mask;

?

Paolo