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From: John Baldwin <jhb@freebsd.org>
To: Mateusz Guzik <mjguzik@gmail.com>
Subject: Re: svn commit: r278320 - in head: contrib/mdocml lib lib/libdevctl
 share/mk sys/dev/acpica sys/dev/pci sys/kern sys/sys usr.sbin usr.sbin/devctl
Date: Thu, 26 Feb 2015 07:46:38 -0500
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On Thursday, February 26, 2015 12:40:06 AM Mateusz Guzik wrote:
> On Fri, Feb 06, 2015 at 04:09:02PM +0000, John Baldwin wrote:
> > Author: jhb
> > Date: Fri Feb  6 16:09:01 2015
> > New Revision: 278320
> > URL: https://svnweb.freebsd.org/changeset/base/278320
> > 
> > Log:
> >   Add a new device control utility for new-bus devices called devctl. 
> >   This
> >   allows the user to request administrative changes to individual devices
> 
> [..]
> 
> > +static int
> > +devctl2_ioctl(struct cdev *cdev, u_long cmd, caddr_t data, int fflag,
> > +    struct thread *td)
> > +{
> > +	struct devreq *req;
> > +	device_t dev;
> > +	int error, old;
> > +
> > +	/* Locate the device to control. */
> > +	mtx_lock(&Giant);
> > +	req = (struct devreq *)data;
> 
> [..]
> 
> > +	switch (cmd) {
> 
> [..]
> 
> > +	case DEV_SET_DRIVER: {
> > +		devclass_t dc;
> > +		char driver[128];
> > +
> > +		error = copyinstr(req->dr_data, driver, sizeof(driver), NULL);
> 
> [..]
> 
> > +		if (!driver_exists(dev->parent, driver)) {
> > +			error = ENOENT;
> > +			break;
> > +		}
> 
> [..]
> 
> > +	}
> > +	mtx_unlock(&Giant);
> > +	return (error);
> > +}
> 
> I only skimmed thourgh this, will not a page fault drop + reacquire Giant
> lock?
> 
> iow, would not it be better to copy prior to taking the lock?

It won't make a difference.  All the logic is done after the copy, so it is 
still atomic.

-- 
John Baldwin