lkml.org 
[lkml]   [2010]   [Jun]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: sequence lock in Linux
* 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


\
 
 \ /
  Last update: 2010-06-11 22:49    [W:0.550 / U:0.248 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site