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

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Date:   Thu, 16 Nov 2017 10:25:46 +0000
From:   Will Deacon <will.deacon@....com>
To:     Daniel Lustig <dlustig@...dia.com>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        Palmer Dabbelt <palmer@...belt.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

Hi Daniel,

On Thu, Nov 16, 2017 at 06:40:46AM +0000, Daniel Lustig wrote:
> > > 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

That presents you with an interesting choice when implementing locks: do you
use .aqrl for lock and unlokc and elide smp_mb__after_spinlock, or do you use
.aq/.rl for lock/unlock respectively and emit a fence for
smp_mb__after_spinlock?

> - .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

Thanks for the update, even this brief summary is better than the current
architecture document ;)

> 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.

Ok, I'll bite! Do you forbid LB?

Will