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From: Peter Zijlstra <peterz@infradead.org>
To: hev <r@hev.cc>
Cc: Will Deacon <will@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
	linux-kernel@vger.kernel.org, stern@rowland.harvard.edu,
	parri.andrea@gmail.com, npiggin@gmail.com, dhowells@redhat.com,
	j.alglave@ucl.ac.uk, luc.maranget@inria.fr, paulmck@kernel.org,
	akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org,
	huacai chen <chenhuacai@gmail.com>, Guo Ren <guoren@kernel.org>,
	geert@linux-m68k.org, Huacai Chen <chenhuacai@loongson.cn>,
	Ingo Molnar <mingo@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
	wangrui <wangrui@loongson.cn>, lixuefeng <lixuefeng@loongson.cn>,
	Jiaxun Yang <jiaxun.yang@flygoat.com>
Subject: Re: [PATCH] Documentation/atomic_t: Document forward progress expectations
Date: Thu, 29 Jul 2021 22:03:29 +0200	[thread overview]
Message-ID: <20210729200329.GC82583@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <CAHirt9hsN9cy16TKSn7Bb+HG5M52FR1Ct8=7xDiM14+5K_S8eg@mail.gmail.com>

On Fri, Jul 30, 2021 at 12:24:14AM +0800, hev wrote:
> We may need new APIs to help LL/SC to implement atomic operations, but
> this is obviously incompatible with native CAS. and many and many
> common functions are CAS friendly. Let's more functions that implement
> atomic semantics can be overridden by architecture may be a way. ;-)
> 
> In the above example, the correct implementation on LL/SC may be like:
> 
> do {
>     old = LL(&v);
>     new = func(old, &skip);
>     if (skip) {
>         break;
>     }
> } while (!SC(&v, new);
> 
> However, the success of SC may be affected by the inconstant
> complexity of func. :-(

Right, so you can't really do that because the architecture constraints
on what is allowed between LL and SC vary. Also, you couldn't compile
that code on a CAS architecture because you simply cannot implement the
LL/SC semantics using CAS.

One thing that can be done is having the compiler transform a CAS loop
into a LL/SC loop, and clang actually tries that, but GCC is absolutely
failing there:

  https://godbolt.org/z/1MK6ceq46

(note; clang only does this for arm64, and the code it does generate is
pretty horrific)

And this is another thing where C11 is utter crap; because as far as
it's concerned this is equivalent, while obviously it is not, per the
parent argument.

Also, ideally there would be a variant where you'd mandate the
forward progress or a compiler error when not possible.

  reply	other threads:[~2021-07-29 20:05 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-29 14:40 Peter Zijlstra
2021-07-29 16:24 ` hev
2021-07-29 20:03   ` Peter Zijlstra [this message]
2021-08-05  9:40 ` [tip: locking/core] " tip-bot2 for Peter Zijlstra

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