linux-kernel - RE: [patches] Re: [PATCH v9 05/12] RISC-V: Atomic and Locking Code

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Message-ID: <2965fc412451432db276d561d47902fb@HQMAIL105.nvidia.com>
Date:   Thu, 16 Nov 2017 06:40:46 +0000
From:   Daniel Lustig <dlustig@...dia.com>
To:     Boqun Feng <boqun.feng@...il.com>
CC:     Palmer Dabbelt <palmer@...belt.com>,
        "will.deacon@....com" <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "patches@...ups.riscv.org" <patches@...ups.riscv.org>,
        "peterz@...radead.org" <peterz@...radead.org>
Subject: RE: [patches] Re: [PATCH v9 05/12] RISC-V: Atomic and Locking Code

> > In that case, maybe we should just start out having a fence on both
> > sides for
> 
> Actually, given your architecture is RCsc rather than RCpc, so I think maybe
> you could follow the way that ARM uses(i.e. relaxed load + release store + a
> full barrier). You can see the commit log of 8e86f0b409a4
> ("arm64: atomics: fix use of acquire + release for full barrier
> semantics") for the reasoning:
> 
> 	https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/co
> mmit/?id=8e86f0b409a44193f1587e87b69c5dcf8f65be67

OK I'm catching up now...and I have to say, that is clever!  Thanks for the
corrections.  It would definitely be good to avoid the double fence.  Let me
think this over a bit more.

One subtle point about RCpc vs. RCsc, though: see below.

> 
> Sounds great! Any estimation when we can see that(maybe a draft)?

The latest should be November 28-30, coinciding with the next RISC-V workshop.
Possibly even sooner.  We just recently resolved a big debate that's been
holding us up for a while, and so now it's just a matter of me writing it all
up so it's understandable.

In the meantime, though, let me quickly and informally summarize some of the
highlights, in case it helps unblock any further review here:

- The model is now (other-)multi-copy atomic
- .aq and .rl alone mean RCpc
- .aqrl means RCsc
- .aq applies only to the read part of an RMW
- .rl applies only to the write part of an RMW
- Syntactic dependencies are now respected

I recognize this isn't enough detail to do it proper justice, but we'll get the
full model out as soon as we can.

> 
> Regards,
> Boqun
> 
> > Dan