Re: [PATCH v7 08/10] tpm: Proxy driver for supporting multiple emulated TPMs

["Stefan Berger" <stefanb-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

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Jarkko Sakkinen <jarkko.sakkinen-VuQAYsv1563Yd54FQh9/CA@public.gmane.org> wrote on 03/13/2016 
04:06:52 PM:


> 
> On Sat, Mar 12, 2016 at 06:27:13PM -0500, Stefan Berger wrote:

> >    This fix should solve the problem:
> > 
> >    diff --git a/drivers/char/tpm/tpm_vtpm_proxy.c
> >    b/drivers/char/tpm/tpm_vtpm_proxy.c
> >    index d73944e..01e5070 100644
> >    --- a/drivers/char/tpm/tpm_vtpm_proxy.c
> >    +++ b/drivers/char/tpm/tpm_vtpm_proxy.c
> >    @@ -456,10 +456,10 @@ err_delete_proxy_dev:
> >      */
> >     static void vtpm_proxy_delete_device(struct proxy_dev *proxy_dev)
> >     {
> >    -       tpm_chip_unregister(proxy_dev->chip);
> >    -
> >            vtpm_proxy_fops_undo_open(proxy_dev);
> > 
> >    +      tpm_chip_unregister(proxy_dev->chip);
> >    +
> >            vtpm_proxy_delete_proxy_dev(proxy_dev);
> >     }
> > 
> >    Can you let me know whether this gets it working for you? I'd 
prepare a
> >    v9.
> 
> So is the deadlock such that:
> 
> * tpm_chip_unregister() tries to write lock ops_sem.
> * tpm_transmit() holds read lock to ops_sem.
> 
> This takes two minutes if no timeouts are calculated.
> 
> And is the effect of moving vtpm_proxy_fops_undo_open() upwards such
> that vtpm_proxy_tpm_req_canceled() starts returning true, which in
> effect breaks the loop in tpm_transmit()?
> 
> Yeah, the fix as a code change is very simple but I had to use perf
> probe to verify this so maybe a comment there would be in place to
> tell why tpm_chip_unregister() must be called last (write lock).

I'll put a comment. The reason is that the client (user of /dev/tpmX) is 
possibly holding the 'ops' lock while we the proxy driver needs it when 
calling tom_chip_unregister.
Thanks for trying it.

   Stefan



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