Re: [PATCH 00/14] alpha: cleanups for 6.10

[Paul E. McKenney]

On Fri, May 31, 2024 at 04:56:28AM +0100, Maciej W. Rozycki wrote:
> On Wed, 29 May 2024, Paul E. McKenney wrote:
> 
> > >  Mind that the read-modify-write sequence that software does for sub-word 
> > > write accesses with original Alpha hardware is precisely what hardware 
> > > would have to do anyway and support for that was deliberately omitted by 
> > > the architecture designers from the ISA to give it performance advantages 
> > > quoted in the architecture manual.  The only difference here is that with 
> > > hardware read-modify-write operations atomicity for sub-word accesses is 
> > > guaranteed by the ISA, however for software read-modify-write it has to be 
> > > explictly coded using the usual load-locked/store-conditional sequence in 
> > > a loop.  I don't think it's a big deal really, it should be trivial to do 
> > > in the relevant accessors, along with the memory barriers that are needed 
> > > anyway for EV56+ and possibly other ports such as the MIPS one.
> > 
> > There shouldn't be any memory barriers required, and don't EV56+ have
> > single-byte loads and stores?
> 
>  I should have commented on this in my original reply.
> 
>  You're the RCU expert so you know the answer.  I don't.  If it's OK for
> successive writes to get reordered, or readers to see a stale value, then 
> you don't need memory barriers.  Otherwise you do.  Whether byte accesses 
> are available or not does not matter, the CPU *will* do reordering if it's 
> allowed to (or more specifically, it won't do anything to prevent it from 
> happening, especially in SMP configurations; I can't remember offhand if 
> there are cases with UP).  Also adjacent byte writes may be merged, but I 
> suppose it does not matter, or does it?

RCU uses whichever wrapper is required.  For example, if ordering is
required, it might use smp_store_release() and smp_load_acquire().
If ordering does not matter, it might use WRITE_ONCE() and READ_ONCE().
If tearing/fusing/merging does not matter, as in there are not concurrent
accesses, it uses plain C-language loads and stores.

>  NB MIPS has similar architectural arrangements (and a bunch of barriers 
> defined in the ISA), it's just most implementations are actually strongly 
> ordered, so most people can't see the effects of this.  With MIPS I know 
> for sure there are cases of UP reordering, but they only really matter for 
> MMIO use cases and not regular memory.

Any given architecture is required to provide architecture-specific
implementations of the various functions that meet the requirements of
Linux-kernel memory model.  See tools/memory-model for more information.

							Thanx, Paul