Re: [External] Re: [PATCH v3 0/8] riscv: Add Zalasr ISA extension support

Andrea Parri
2025-09-19T11:06:23Z

> > > (not a review, just looking at this diff stat) is changing the fastpath
> > >
> > > read_unlock()
> > > read_lock()
> > >
> > > from something like
> > >
> > > fence rw,w
> > > amodadd.w
> > > amoadd.w
> > > fence r,rw
> > >
> > > to
> > >
> > > fence rw,rw
> > > amoadd.w
> > > amoadd.w
> > > fence rw,rw
> > >
> > > no matter Zalasr or !Zalasr. Similarly for other atomic operations with
> > > release or acquire semantics. I guess the change was not intentional?
> > > If it was intentional, it should be properly mentioned in the changelog.
> >
> > Sorry about that. It is intended. The atomic operation before
> > __atomic_acquire_fence or operation after __atomic_release_fence can
> > be just a single ld or sd instruction instead of amocas or amoswap. In
> > such cases, when the store release operation becomes 'sd.rl', the
> > __atomic_acquire_fence via 'fence r, rw' can not ensure FENCE.TSO
> > anymore. Thus I replace it with 'fence rw, rw'.

But you could apply similar changes you performed for xchg & cmpxchg: use
.AQ and .RL for other atomic RMW operations as well, no? AFAICS, that is
what arm64 actually does in arch/arm64/include/asm/atomic_{ll_sc,lse}.h .

Andrea

> This is also the common implementation on other architectures who use
> aq/rl instructions like ARM. And you certainly already knew it~