Re: [Qemu-devel] [PATCH] ide: fix double free

[ChenLiang <chenliang88@huawei.com>]

On 2014/7/2 18:16, Paolo Bonzini wrote:

> Il 02/07/2014 11:46, Gonglei (Arei) ha scritto:
>> Hi, Paolo. We have tested your above patch, and it works well for us.
> 
> I'm still not sure where the fix is.  I jotted the patch quickly, but I'd rather understand it better before submitting it.  Here is it again:
> 
> --- a/dma-helpers.c
> +++ b/dma-helpers.c
> @@ -181,15 +181,15 @@ static void dma_aio_cancel(BlockDriverAIOCB *acb)
>      trace_dma_aio_cancel(dbs);
> 
> +    dbs->in_cancel = true;
>      if (dbs->acb) {
>          BlockDriverAIOCB *acb = dbs->acb;
>          dbs->acb = NULL;
> -        dbs->in_cancel = true;
>          bdrv_aio_cancel(acb);
> -        dbs->in_cancel = false;
>      }
>      dbs->common.cb = NULL;
>      dma_complete(dbs, 0);
> +    qemu_aio_release(dbs);
>  }
> 


Hmm, dbs->in_cancel will be true always. Although this will avoid freeing dbs by dma_comlete.
But it maybe a mistake.

>  static const AIOCBInfo dma_aiocb_info = {
> 
> and here is dma_complete:
> 
> static void dma_complete(DMAAIOCB *dbs, int ret)
> {
>     trace_dma_complete(dbs, ret, dbs->common.cb);
>     dma_bdrv_unmap(dbs);
>     if (dbs->common.cb) {
>         dbs->common.cb(dbs->common.opaque, ret);
>     }
>     qemu_iovec_destroy(&dbs->iov);
>     if (dbs->bh) {
>         qemu_bh_delete(dbs->bh);
>         dbs->bh = NULL;
>     }
>     if (!dbs->in_cancel) {
>         /* Requests may complete while dma_aio_cancel is in progress.
>          * this case, the AIOCB should not be released because it is
>          * still referenced by dma_aio_cancel.  */
>         qemu_aio_release(dbs);
>     }
> }
> 
> The qemu_aio_release call obviously happened during the second dma_complete call.
> 
> However, the second dma_complete call does not call the callback, and even if dma_bdrv_unmap resulted in a call to continue_after_map_failure, the bottom half would be cancelled immediately.  So we changed from:
> 
>     dbs->in_cancel = false;
> 
>     /* dma_complete calls dma_bdrv_unmap */
>     for (i = 0; i < dbs->iov.niov; ++i) {
>         /* might schedule dbs->bh */
>         dma_memory_unmap(dbs->sg->as, dbs->iov.iov[i].iov_base,
>                          dbs->iov.iov[i].iov_len, dbs->dir,
>                          dbs->iov.iov[i].iov_len);
>     }
>     qemu_iovec_reset(&dbs->iov);
> 
>     /* back to dma_complete, dbs->common.cb == NULL */
>     qemu_iovec_destroy(&dbs->iov);
>     if (dbs->bh) {
>         /* dbs->bh now unscheduled, so it will never run */
>         qemu_bh_delete(dbs->bh);
>         dbs->bh = NULL;
>     }
>     /* dbs->in_cancel is false here.  */
>     if (!dbs->in_cancel) {
>         /* Requests may complete while dma_aio_cancel is in progress.
>          * this case, the AIOCB should not be released because it is
>          * still referenced by dma_aio_cancel.  */
>         qemu_aio_release(dbs);
>     }
> 
> to
> 
>     dbs->in_cancel = false;
> 
>     /* dma_complete calls dma_bdrv_unmap */
>     for (i = 0; i < dbs->iov.niov; ++i) {
>         /* might schedule dbs->bh */
>         dma_memory_unmap(dbs->sg->as, dbs->iov.iov[i].iov_base,
>                          dbs->iov.iov[i].iov_len, dbs->dir,
>                          dbs->iov.iov[i].iov_len);
>     }
>     qemu_iovec_reset(&dbs->iov);
> 
>     /* back to dma_complete, dbs->common.cb == NULL */
>     qemu_iovec_destroy(&dbs->iov);
>     if (dbs->bh) {
>         /* dbs->bh now unscheduled, so it will never run */
>         qemu_bh_delete(dbs->bh);
>         dbs->bh = NULL;
>     }
> 
>     /* after the patch dbs->in_cancel is true, the if never triggers */
>     if (!dbs->in_cancel) {
>         /* Requests may complete while dma_aio_cancel is in progress.
>          * this case, the AIOCB should not be released because it is
>          * still referenced by dma_aio_cancel.  */
>         qemu_aio_release(dbs);
>     }
> 
>     /* back to dma_aio_cancel */
>     qemu_aio_release(dbs);
> 
> This doesn't make sense to me.  Can you find out where I'm wrong?
> 
> Paolo
> 
> .
>