linux-kernel - Re: [PATCH v7 03/15] s390/zcrypt: driver callback to indicate resource in use

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Date:   Thu, 16 Apr 2020 11:33:56 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Harald Freudenberger <freude@...ux.ibm.com>
Cc:     Tony Krowiak <akrowiak@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        borntraeger@...ibm.com, mjrosato@...ux.ibm.com,
        pmorel@...ux.ibm.com, pasic@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        jjherne@...ux.ibm.com, fiuczy@...ux.ibm.com
Subject: Re: [PATCH v7 03/15] s390/zcrypt: driver callback to indicate
 resource in use

On Wed, 15 Apr 2020 08:08:24 +0200
Harald Freudenberger <freude@...ux.ibm.com> wrote:

> On 14.04.20 14:58, Cornelia Huck wrote:
> > On Tue,  7 Apr 2020 15:20:03 -0400
> > Tony Krowiak <akrowiak@...ux.ibm.com> wrote:

> >> +	/* The non-default driver's module must be loaded */
> >> +	if (!try_module_get(drv->owner))
> >> +		return 0;  
> > Is that really needed? I would have thought that the driver core's
> > klist usage would make sure that the callback would not be invoked for
> > drivers that are not registered anymore. Or am I missing a window?  
> The try_module_get() and module_put() is a result of review feedback from
> my side. The ap bus core is static in the kernel whereas the
> vfio dd is a kernel module. So there may be a race condition between
> calling the callback function and removal of the vfio dd module.
> There is similar code in zcrypt_api which does the same for the zcrypt
> device drivers before using some variables or functions from the modules.
> Help me, it this is outdated code and there is no need to adjust the
> module reference counter any more, then I would be happy to remove
> this code :-)

I think the driver core already should keep us safe. A built-in bus
calling a driver in a module is a very common pattern, and I think
->owner was introduced exactly for that case.

Unless I'm really missing something obvious?