Re: [Qemu-devel] [PATCH v2 4/4] dma-helpers: Move reschedule_dma BH to blk's AioContext

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/03/2015 11:48, Paolo Bonzini wrote:
> > The other possibility is grab a reference for the cpu_register_map_client call,
> > and release it in reschedule_dma. This way the atomics can keep, but we'll need
> > a "finished" flag in DMAAIOCB to avoid double completion.
> Considering this is a slow path, a lock seems preferrable.

And another problem...

You need to be careful about dma_aio_cancel running together with the
continue_after_map_failure, because continue_after_map_failure can be
called by another thread.  You could have

     continue_after_map_failure               dma_aio_cancel
     ------------------------------------------------------------------
     aio_bh_new
                                              qemu_bh_delete
     qemu_bh_schedule (use after free)

To fix this, my suggestion is to pass a BH directly to
cpu_register_map_client (possibly to cpu_unregister_map_client as well?
 seems to have pros and cons).  Then cpu_notify_clients can run entirely
with the lock taken, and not race against cpu_unregister_map_client.
dma_aio_cancel can just do cpu_unregister_map_client followed by
qemu_bh_delete.

Paolo