On Fri, Feb 10, 2023 at 09:32:37AM -0500, Stefan Hajnoczi wrote: > dma_blk_cb() only takes the AioContext lock around ->io_func(). That > means the rest of dma_blk_cb() is not protected. In particular, the > DMAAIOCB field accesses happen outside the lock. > > There is a race when the main loop thread holds the AioContext lock and > invokes scsi_device_purge_requests() -> bdrv_aio_cancel() -> > dma_aio_cancel() while an IOThread executes dma_blk_cb(). The dbs->acb > field determines how cancellation proceeds. If dma_aio_cancel() see
"sees" or "can see" > dbs->acb == NULL while dma_blk_cb() is still running, the request can be > completed twice (-ECANCELED and the actual return value). > > The following assertion can occur with virtio-scsi when an IOThread is > used: > > ../hw/scsi/scsi-disk.c:368: scsi_dma_complete: Assertion `r->req.aiocb != > NULL' failed. > > Fix the race by holding the AioContext across dma_blk_cb(). Now > dma_aio_cancel() under the AioContext lock will not see > inconsistent/intermediate states. > > Cc: Paolo Bonzini <pbonz...@redhat.com> > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> > --- > hw/scsi/scsi-disk.c | 4 +--- > softmmu/dma-helpers.c | 12 +++++++----- > 2 files changed, 8 insertions(+), 8 deletions(-) Widening the scope protected by the lock makes sense, insofar as we still need the lock. Reviewed-by: Eric Blake <ebl...@redhat.com> -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
