Re: sequence lock in Linux

From: Paul E. McKenney
Date: Fri Jun 11 2010 - 16:07:10 EST

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
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