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

[Paolo Bonzini]

Il 02/07/2014 13:33, ChenLiang ha scritto:
> 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.

This was on purpose; I'm doing the free myself in dma_aio_cancel, so I 
wanted to avoid the qemu_aio_release from dma_complete.  This was in 
case of a recursive call to dma_complete.  But I don't see how that 
recursive call could happen outside the "if (dbs->acb)"; and inside the 
"if" the protection is there already.

Can you gather the backtraces for _both_ calls to qemu_aio_release, 
rather than just the second?

With what guest are you encountering the bug?

Paolo