linux-kernel - Re: CPU ordering with respect to krefs

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Message-Id: <200704170744.49157.oneukum@suse.de>
Date:	Tue, 17 Apr 2007 07:44:48 +0200
From:	Oliver Neukum <oneukum@...e.de>
To:	Greg KH <greg@...ah.com>
Cc:	Eric Dumazet <dada1@...mosbay.com>, Greg KH <gregkh@...e.de>,
	linux-kernel@...r.kernel.org
Subject: Re: CPU ordering with respect to krefs

Am Donnerstag, 12. April 2007 08:27 schrieb Greg KH:
> On Mon, Apr 02, 2007 at 04:33:54PM +0200, Eric Dumazet wrote:
> > On Mon, 2 Apr 2007 14:47:59 +0200
> > Oliver Neukum <oneukum@...e.de> wrote:
> > 
> > > Hi,
> > > 
> > > some atomic operations are only atomic, not ordered. Thus a CPU is allowed
> > > to reorder memory references to an object to before the reference is
> > > obtained. This fixes it.
> > > 
> > > 	Regards
> > > 		Oliver
> > > Signed-off-by: Oliver Neukum <oneukum@...e.de>
> > > ------
> > > 
> > > --- a/lib/kref.c	2007-04-02 14:40:40.000000000 +0200
> > > +++ b/lib/kref.c	2007-04-02 14:40:50.000000000 +0200
> > > @@ -21,6 +21,7 @@
> > >  void kref_init(struct kref *kref)
> > >  {
> > >  	atomic_set(&kref->refcount,1);
> > > +	smp_mb();
> > >  }
> > 
> > I dont understand why smp_mb() is needed here, and not in
> > spinlock_init() for example.
> 
> I think, after reading the Documentation/memory-barriers.txt and
> Documentation/atomic_ops.txt documentation, that spin_lock_init() also
> needs this kind of memory barrier.

spin_lock_init() is not an atomic operation.
In principle, the issue exists. However, the whole issue is a bit of a grey
area. You might take the viewpoint that upping the refcount needs to be
under lock, which needs to take care of ordering issues in case of krefs.
A new spinlock has the same issue. You need to be careful making them
accessible to other CPUs.

If you take code like:

static int producer()
{
	...
	data = kmalloc(...);
	spin_lock_init(&data->lock);
	data->value = some_value;
	data->next = global_pointer;

	global_pointer = data;
	...
}
	
You have an ordering bug anyway, which you can't fix in spin_lock_init().

	Regards
		Oliver
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