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Re: [PATCH 1/3] tpm: Hold the kref during tpm_chip_find_get

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On Sun, Feb 14, 2016 at 06:55:12AM +0200, Jarkko Sakkinen wrote:
> On Fri, Feb 12, 2016 at 05:04:29PM -0700, Jason Gunthorpe wrote:
> > This was missed during the struct device conversion, we
> > need to hold a kref on the chip to make sure it isn't freed.
> > 
> > Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
> 
> I'm bit confused about this patch. What is the regression if this
> needs

The patch is simply totally broken, the placement of the get_device is
wrong:

> > @@ -53,6 +53,8 @@ struct tpm_chip *tpm_chip_find_get(int chip_num)
> >  			chip = pos;
> >  			break;
> >  		}
> > +
> > +		get_device(&chip->dev);

It needs to be moved up two lines before the break, into the if
statement.

As for the urgency - today the tpm core relies on module locking to
try and prevent tpm_chip_unregister from racing with stuff like the
above. That is totally broken in modern kernels, but it is what the
core tries to do. Within that framework the get/put are not needed
because of the module locking.

The only time these additional get/put do anything is when we are
racing with tpm_unregister, but if we are racing with unregister then
there are much bigger problems and things will crash anyhow.

So, this patch is just a tiny step.

The revised version of this patch with the rw_sem attempts to address
the complete race.

Jason

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[PATCH 0/3] Various struct device cleanups Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-13 01:10 +0100
  [PATCH 1/3] tpm: Hold the kref during tpm_chip_find_get Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-13 01:10 +0100
    Re: [PATCH 1/3] tpm: Hold the kref during tpm_chip_find_get Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> - 2016-02-13 11:10 +0100
      Re: [PATCH 1/3] tpm: Hold the kref during tpm_chip_find_get Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-13 16:50 +0100
    Re: [PATCH 1/3] tpm: Hold the kref during tpm_chip_find_get Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> - 2016-02-14 06:00 +0100
      Re: [PATCH 1/3] tpm: Hold the kref during tpm_chip_find_get Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-14 08:00 +0100
        Re: [PATCH 1/3] tpm: Hold the kref during tpm_chip_find_get Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> - 2016-02-14 09:10 +0100
  [PATCH 3/3] tpm: Get rid of devname Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-13 01:10 +0100
    Re: [PATCH 3/3] tpm: Get rid of devname Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-13 02:20 +0100
    Re: [PATCH 3/3] tpm: Get rid of devname Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> - 2016-02-14 06:20 +0100
  [PATCH 2/3] tpm: Get rid of chip->pdev Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-13 01:10 +0100
    Re: [PATCH 2/3] tpm: Get rid of chip->pdev Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-13 02:20 +0100
      Re: [PATCH 2/3] tpm: Get rid of chip->pdev Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-13 03:10 +0100
      Re: [PATCH 2/3] tpm: Get rid of chip->pdev Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-13 04:40 +0100
        Re: [PATCH 2/3] tpm: Get rid of chip->pdev Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> - 2016-02-14 06:30 +0100
          Re: [PATCH 2/3] tpm: Get rid of chip->pdev Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-14 08:00 +0100
            Re: [PATCH 2/3] tpm: Get rid of chip->pdev Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> - 2016-02-14 09:10 +0100
    Re: [tpmdd-devel] [PATCH 2/3] tpm: Get rid of chip->pdev Stefan Berger <stefanb@linux.vnet.ibm.com> - 2016-02-13 16:50 +0100
      Re: [tpmdd-devel] [PATCH 2/3] tpm: Get rid of chip->pdev Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2016-02-14 08:10 +0100
    Re: [PATCH 2/3] tpm: Get rid of chip->pdev Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> - 2016-02-14 06:20 +0100

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