On Fri, Jul 08, 2022 at 11:01:37AM +0200, Emanuele Giuseppe Esposito wrote: > > > Am 05/07/2022 um 16:11 schrieb Stefan Hajnoczi: > > On Thu, Jun 09, 2022 at 10:37:20AM -0400, Emanuele Giuseppe Esposito wrote: > >> @@ -146,7 +147,6 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev) > >> > >> s->dataplane_starting = false; > >> s->dataplane_started = true; > >> - aio_context_release(s->ctx); > >> return 0; > > > > This looks risky because s->dataplane_started is accessed by IO code and > > there is a race condition here. Maybe you can refactor the code along > > the lines of virtio-blk to avoid the race. > > > > Uhmm could you explain why is virtio-blk also safe here? > And what is currently protecting dataplane_started (in both blk and > scsi, as I don't see any other AioContext lock taken)?
dataplane_started is assigned before the host notifier is set up, which I'm assuming is an implicit write barrier. > Because I see that for example virtio_blk_req_complete is IO_CODE, so it > could theoretically read dataplane_started while it is being changed in > dataplane_stop? Even though I guess it doesn't because we disable and > clean the host notifier before modifying it? virtio_blk_data_plane_stop() has: aio_context_acquire(s->ctx); aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s); /* Drain and try to switch bs back to the QEMU main loop. If other users * keep the BlockBackend in the iothread, that's ok */ blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context(), NULL); aio_context_release(s->ctx); and disables host notifiers. At that point the IOThread no longer receives virtqueue kicks and all in-flight requests have completed. dataplane_started is only written afterwards so there is no race with virtio_blk_req_complete(). > > But if so, I don't get what is the difference with scsi code, and why we > need to protect only that instance with the aiocontext lock? The race condition I pointed out is not with virtio_blk_req_complete() and data_plane_stop(). It's data_plane_start() racing with virtio_blk_req_complete(). The virtio-scsi dataplane code is different for historical reasons and happens to have the race. I don't think the virtio-blk code is affected. Stefan
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