linux-kernel - Re: [PATCH] drivers/isdn/gigaset: new M101 driver

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Date:	Sun, 4 Feb 2007 20:58:17 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tilman Schmidt <tilman@...p.cc>
Cc:	Karsten Keil <kkeil@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	i4ldeveloper@...tserv.isdn4linux.de, linux-serial@...r.kernel.org,
	Hansjoerg Lipp <hjlipp@....de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH] drivers/isdn/gigaset: new M101 driver

On Mon, 05 Feb 2007 02:42:09 +0100 Tilman Schmidt <tilman@...p.cc> wrote:

> Am 04.02.2007 02:56 schrieb Andrew Morton:
> > On Sun, 04 Feb 2007 02:32:41 +0100 Tilman Schmidt <tilman@...p.cc> wrote:
> > 
> >>>> +	spin_lock_irqsave(&cs->cmdlock, flags);
> >>>> +	cb = cs->cmdbuf;
> >>>> +	spin_unlock_irqrestore(&cs->cmdlock, flags);
> >>> It is doubtful if the locking here does anything useful.
> >> It assures atomicity when reading the cs->cmdbuf pointer.
> > 
> > I think it's bogus.  If the quantity being copied here is more than 32-bits
> > then yes, a lock is appropriate.  But if it's a single word then it's
> > unlikely that the locking does anything useful.  Or there might be a bug
> > here.
> 
> It's a pointer. Are reads and writes of pointer sized objects
> guaranteed to be atomic on every platform?

Yup - we make the same assumption about longs in various places.

It's a bit strange to read a pointer which can be changing at the
same time.  Because the local copy will no longer represent the
thing which it was just copied from.
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