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.
i'll look at it a bit more carefully.
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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