linux-kernel - Re: [PATCH 4.4 40/57] tpm: Provide strong locking for device removal

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Date:   Wed, 26 Jul 2017 12:56:37 -0700
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Ben Hutchings <ben.hutchings@...ethink.co.uk>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Subject: Re: [PATCH 4.4 40/57] tpm: Provide strong locking for device removal

On Tue, Jul 25, 2017 at 11:56:01PM +0100, Ben Hutchings wrote:
> On Wed, 2017-07-19 at 13:12 +0200, Greg Kroah-Hartman wrote:
> > 4.4-stable review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Jason Gunthorpe <jgunthorpe@...idianresearch.com>
> > 
> > commit 4e26195f240d73150e8308ae42874702e3df8d2c upstream.
> > 
> > Add a read/write semaphore around the ops function pointers so
> > ops can be set to null when the driver un-registers.
> [...]
> > @@ -49,10 +99,10 @@ struct tpm_chip *tpm_chip_find_get(int c
> >  		if (chip_num != TPM_ANY_NUM && chip_num != pos->dev_num)
> >  			continue;
> >  
> > -		if (try_module_get(pos->dev.parent->driver->owner)) {
> > +		/* rcu prevents chip from being free'd */
> > +		if (!tpm_try_get_ops(pos))
> [...]
> 
> But an RCU read-side critical section is an atomic context, and
> semaphore operations can block!  Fixed upstream by:
> 
> commit 15516788e581eb32ec1c50e5f00aba3faf95d817
> Author: Stefan Berger <stefanb@...ux.vnet.ibm.com>
> Date:   Mon Feb 29 08:53:02 2016 -0500
> 
>     tpm: Replace device number bitmap with IDR

Ugh, that's a big patch.

Jason, Stefan, and Jarkko, what do you think?  Should I also take this
for 4.4-stable?

thanks,

greg k-h