On 03/01/2017 01:57 PM, Paolo Bonzini wrote: > > > On 01/03/2017 12:50, Halil Pasic wrote: >> The commits 03de2f527 "virtio-blk: do not use vring in dataplane" and >> 9ffe337c08 "virtio-blk: always use dataplane path if ioeventfd is active" >> changed how notifications are done for virtio-blk substantially. Due to a >> race condition interrupts are lost when irqfd is torn down after >> notify_guest_bh was scheduled but before it actually runs. > > I don't think the non-irqfd notification mechanism is thread safe, and > that would be a problem for this patch.
Sorry, haven't looked into that thoroughly (and speculated people with more understanding will jump in). > > What is the path that causes the irqfd to be torn down? Only something Here a trace: 135871@1488304024.512533:virtio_blk_req_complete req 0x2aa6b117e10 status 0 135871@1488304024.512541:virtio_notify_irqfd vdev 0x2aa6b0e19d8 vq 0x2aa6b4c0870 135871@1488304024.522607:virtio_blk_req_complete req 0x2aa6b118980 status 0 135871@1488304024.522616:virtio_blk_req_complete req 0x2aa6b119260 status 0 135871@1488304024.522627:virtio_notify_irqfd vdev 0x2aa6b0e19d8 vq 0x2aa6b4c0870 135871@1488304024.527386:virtio_blk_req_complete req 0x2aa6b118980 status 0 135871@1488304024.527431:virtio_notify_irqfd vdev 0x2aa6b0e19d8 vq 0x2aa6b4c0870 135871@1488304024.528611:virtio_guest_notifier_read vdev 0x2aa6b0e61c8 vq 0x2aa6b4de880 135871@1488304024.528628:virtio_guest_notifier_read vdev 0x2aa6b0e61c8 vq 0x2aa6b4de8f8 135871@1488304024.528753:virtio_blk_data_plane_stop dataplane 0x2aa6b0e5540 ^== DATAPLANE STOP 135871@1488304024.530709:virtio_blk_req_complete req 0x2aa6b117e10 status 0 135871@1488304024.530752:virtio_guest_notifier_read vdev 0x2aa6b0e19d8 vq 0x2aa6b4c0870 ^== comes from k->set_guest_notifiers(qbus->parent, nvqs, false); in virtio_blk_data_plane_stop and done immediately after irqfd is cleaned up by the transport 135871@1488304024.530836:virtio_notify_irqfd vdev 0x2aa6b0e19d8 vq 0x2aa6b4c0870 halil: error in event_notifier_set: Bad file descriptor ^== here we have the problem If you want a stacktrace that can be arranged to. > like a reset should cause it (the only call in virtio-blk is from > virtio_blk_data_plane_stop), and then the guest doesn't care anymore > about interrupts. I do not understand this with 'doesn't care anymore about interrupts'. I was debugging a virtio-blk device being stuck waiting for a host notification (interrupt) after migration. > > That path also does a qemu_bh_delete, so the notify_guest_bh should not > be invoked at all. > That's only for destroy. I'm migrating. Seems I tried to fix this is the wrong way. Was not too confident about it in the first place. Suggestions welcome! Cheers, Halil