Re: sequence lock in Linux

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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?

							Thanx, Paul