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

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Message-ID: <aM05J6FU0xG3SBzR@andrea>
Date: Fri, 19 Sep 2025 13:06:15 +0200
From: Andrea Parri <parri.andrea@...il.com>
To: Xu Lu <luxu.kernel@...edance.com>
Cc: corbet@....net, robh@...nel.org, krzk+dt@...nel.org,
	conor+dt@...nel.org, paul.walmsley@...ive.com, palmer@...belt.com,
	aou@...s.berkeley.edu, alex@...ti.fr, will@...nel.org,
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	atish.patra@...ux.dev, pbonzini@...hat.com, shuah@...nel.org,
	devicetree@...r.kernel.org, linux-riscv@...ts.infradead.org,
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	kvm-riscv@...ts.infradead.org, linux-kselftest@...r.kernel.org
Subject: Re: [External] Re: [PATCH v3 0/8] riscv: Add Zalasr ISA extension
 support

> > > (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~