linux-kernel - Re: Incorrect uses of get_driver()/put

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Date:	Mon, 9 Jan 2012 14:36:33 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Joerg Roedel <joerg.roedel@....com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Michael Buesch <m@...s.ch>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Incorrect uses of get_driver()/put_driver()

On Mon, 9 Jan 2012, Dmitry Torokhov wrote:

> > > > > lib/dma-debug.c:173:  drv = get_driver(dev->driver);
> > > > > lib/dma-debug.c:188:  put_driver(drv);
> > > > > 
> > > > > Joerg, these calls don't seem to do anything, as far as I can tell.  
> > > > > Is there any reason to keep them?
> > > > > 
> > > > > drivers/pci/xen-pcifront.c:596:       if (get_driver(&pdrv->driver)) {
> > > > > drivers/pci/xen-pcifront.c:626:               put_driver(&pdrv->driver);
> > > > > 
> > > > > Konrad, these calls don't seem to do anything either.
> > > > > 
> > > > 
> > > > Looks like they should be replaced with the try_module_get() equivalant
> > > > for the 'struct pci_driver'? Is there such one?
> > > 
> > > You seem to need stronger guarantees that the driver simply present in
> > > memory. You need to make sure that the driver you fetched is kept being
> > > bound to the device for entire duration of pcifront_common_process().
> > 
> > OK, any suggestions?
> 
> Nothing canned I'm afraid...

device_lock(&pcidev->dev) will block unbinding.  If you take the lock 
before looking at pcidev->driver, it should be okay.

The drawback is that pdrv->error_handler may end up doing something 
that takes the same lock.  If you can verify that won't happen, there 
won't be any problem.

Alan Stern

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