Messages in this thread | | | Date | Fri, 11 Jun 2010 16:46:48 -0400 | From | Mathieu Desnoyers <> | Subject | Re: sequence lock in Linux |
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* 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
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