Re: sequence lock in Linux

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Fri, Jun 11, 2010 at 03:40:16PM -0400, Mathieu Desnoyers wrote:
> > Hi Paul,
> > 
> > (CCing lkml)
> > 
> > Is it just me, or the following code:
> > 
> > static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
> > {
> >         unsigned ret;
> > 
> > repeat:
> >         ret = sl->sequence;
> >         smp_rmb();
> >         if (unlikely(ret & 1)) {
> >                 cpu_relax();
> >                 goto repeat;
> >         }
> > 
> >         return ret;
> > }
> > 
> > could use a ACCESS_ONCE() around the sl->sequence read ? I'm concerned about the
> > compiler generating code that reads the sequence number chunkwise.
> > 
> > The same apply to all other reads of the sequence number in seqlock.h (including
> > the retry code).
> > 
> > Thoughts ?
> 
> Doesn't gcc guarantee that accesses to aligned basic types that fit into
> a machine word are loaded and stored in one shot?  Now, gcc might choose
> to load twice (or to merge loads) due to things like register pressure,
> but given that ->sequence is an int, gcc should not be accessing it
> (say) bytewise on any platform supporting 32-bit accesses.
> 
> Or am I suffering from wishful thinking here?

Hopefully not. I might be the one suffering from extreme compiler distrust here.
;-)

Mathieu

> 
> 							Thanx, Paul

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com