On Tue, 1 Apr 2008, Matthias Kaehlcke wrote:
> El Tue, Apr 01, 2008 at 07:38:05AM -0400 Robert P. J. Day ha dit:
>
> >
> > regarding the routine "chrdev_open()" in fs/char_dev.c:
> >
> > On Tue, 1 Apr 2008, Matthias Kaehlcke wrote:
> >
> > > El Mon, Mar 31, 2008 at 10:10:41PM -0400 Robert P. J. Day ha dit:
> >
> > > > p.s. i am a bit confused as to why there are two tests for whether
> > > > inode->i_cdev is non-NULL:
> > > >
> > > > p = inode->i_cdev;
> > > > if (!p) { <-- first test of p
> > > > struct kobject *kobj;
> > > > int idx;
> > > > spin_unlock(&cdev_lock);
> > > > kobj = kobj_lookup(cdev_map, inode->i_rdev, &idx);
> > > > if (!kobj)
> > > > return -ENXIO;
> > > > new = container_of(kobj, struct cdev, kobj);
> > > > spin_lock(&cdev_lock);
> > > > p = inode->i_cdev;
> > > > if (!p) { <-- second test of p
> > > > inode->i_cdev = p = new;
> > > >
> > > >
> > > > why is that first test being repeated further down? if p was NULL
> > > > at that first test, how could it possibly have changed before that
> > > > second test? isn't that second test redundant? or am i missing
> > > > something?
> > >
> > > if i understand that piece of code correctly inode->i_cdev could
> > > have changed (probably by a concurrent invocation of the same
> > > function) while cdev_lock is not hold and inode->i_cdev is
> > > reassigned to p.
> >
> > possibly, but wouldn't that create an ugly synchronization
> > scenario? say, if two invocations of that routine both arrived at
> > that second test of "p" simultaneously, both found it NULL, and
> > both then tried to set it? and, in any event, both would
> > technically need to set it to the *same* value, anyway. so it's
> > still not clear what that second test is for.
>
> they can't arrive at the second test simultaneously cause cdev_lock
> is used for synchronization.
>
> yes, they would set it to the same value, but two calls to
> list_add(&inode->i_devices, &p->list) would result in
> inode->i_devices being added twice to p->list.
yes, of course you're right, but i still can't help feeling that
there's some unnecessary testing happening here. oh, well ... it
obviously works so i'm not going to agonize over it.
rday
--
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry:
Have classroom, will lecture.
http://crashcourse.ca Waterloo, Ontario, CANADA
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