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

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Date:	Mon, 2 Apr 2007 16:33:54 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: CPU ordering with respect to krefs

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.

If you have ordering issues, then the caller of kref_init() should take care of it, not kref_init() itself.

Random example taken in drivers/usb/gadget/file_storage.c :

static int __init fsg_alloc(void)
{
...
kref_init(&fsg->ref);
init_completion(&fsg->thread_notifier);

the_fsg = fsg;
}

In this example, "the_fsg = fsg" memory write might be visible before the memory writes done in init_completion().
Doing a smp_mb() in kref_init() wont help.

AFAIK kref implementation doesnt need this extra smp_mb().

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