linux-kernel - Re: [RFC] Bridging the gap between the Linux Kernel Memory Consistency Model (LKMM) and C11/C++11 atomics

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Message-ID: <58fa5c39-1481-58f3-9309-aa03bd3344ce@efficios.com>
Date:   Wed, 5 Jul 2023 09:16:50 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Boqun Feng <boqun.feng@...il.com>,
        Olivier Dion <odion@...icios.com>
Cc:     rnk@...gle.com, Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, gcc@....gnu.org, llvm@...ts.linux.dev
Subject: Re: [RFC] Bridging the gap between the Linux Kernel Memory
 Consistency Model (LKMM) and C11/C++11 atomics

On 7/5/23 03:05, Boqun Feng wrote:
> On Mon, Jul 03, 2023 at 03:20:31PM -0400, Olivier Dion wrote:
> [...]
>> NOTE: On x86-64, we found at least one corner case [7] with Clang where
>> a RELEASE exchange is optimized to a RELEASE store, when the returned
>> value of the exchange is unused, breaking the above expectations.
>> Although this type of optimization respect the standard "as-if"
>> statement, we question its pertinence since a user should simply do a
>> RELEASE store instead of an exchange in that case.  With the
>> introduction of these new primitives, these type of optimizations should
>> be revisited.
>>
> 
> FWIW, this is actually a LLVM bug:
> 
> 	https://github.com/llvm/llvm-project/issues/60418

So it was more than a dubious optimization, it's actually broken as well.

I am worried about adding to the compiler's ability to optimize those 
atomics because of the subtle corner-cases/bugs that can creep up.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com