Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <58fa5c39-1481-58f3-9309-aa03bd3344ce@efficios.com>
Date:   Wed, 5 Jul 2023 09:16:50 -0400
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.12.0
Subject: Re: [RFC] Bridging the gap between the Linux Kernel Memory
 Consistency Model (LKMM) and C11/C++11 atomics
Content-Language: en-US
To:     Boqun Feng <boqun.feng@gmail.com>,
        Olivier Dion <odion@efficios.com>
Cc:     rnk@google.com, Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org, gcc@gcc.gnu.org, llvm@lists.linux.dev
References: <87ttukdcow.fsf@laura> <ZKUWPKWAnjZvqsgL@Boquns-Mac-mini.local>
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
In-Reply-To: <ZKUWPKWAnjZvqsgL@Boquns-Mac-mini.local>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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