Re: [PATCH] State limits to safety of _safe iterators

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 13 Sep 2007 08:21:07 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Thu, Sep 13, 2007 at 02:22:45AM -0700, Matthew Helsley wrote:
> > On Wed, 2007-09-12 at 18:01 -0700, Paul E. McKenney wrote:
> > > The _safe list iterators make a blanket statement about how they are
> > > safe against removal.  This patch, inspired by private conversations
> > > with people who unwisely but perhaps understandably took this blanket
> > > statement at its word, adds comments stating limits to this safety.
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > ---
> > > 
> > >  list.h |   42 ++++++++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 42 insertions(+)
> > > 
> > > diff -urpNa -X dontdiff linux-2.6.22/include/linux/list.h linux-2.6.22-safedoc/include/linux/list.h
> > > --- linux-2.6.22/include/linux/list.h	2007-07-08 16:32:17.000000000 -0700
> > > +++ linux-2.6.22-safedoc/include/linux/list.h	2007-09-12 17:45:38.000000000 -0700
> > > @@ -472,6 +472,12 @@ static inline void list_splice_init_rcu(
> > >   * @pos:	the &struct list_head to use as a loop cursor.
> > >   * @n:		another &struct list_head to use as temporary storage
> > >   * @head:	the head for your list.
> > > + *
> > > + * Please note that this is safe only against removal by the code in
> > 
> > I'm not trying to be snarky but how far should we go before expecting
> > folks to read the macros? Depending on the answer you may also want to
> > mention that without additional additional code it's safe only against
> > removal of the list element at pos.
> 
> Good question.  In fact, I would have agreed with you before coming
> across people who in my experience are generally reasonably well clued
> in who were confused about this.
> 

hmm, yes, I must say, one would need to be fairly thick to expect a little
helper macro to protect you from activity on other CPUs.