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: <0c0ba856-a4ba-7133-e751-3a81c82311a9@marcan.st>
Date:   Tue, 16 Aug 2022 17:01:39 +0900
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0)
 Gecko/20100101 Thunderbird/91.12.0
Subject: Re: [PATCH] workqueue: Fix memory ordering race in queue_work*()
Content-Language: en-US
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Will Deacon <will@kernel.org>, Tejun Heo <tj@kernel.org>,
        peterz@infradead.org, jirislaby@kernel.org, maz@kernel.org,
        mark.rutland@arm.com, boqun.feng@gmail.com,
        catalin.marinas@arm.com, oneukum@suse.com,
        roman.penyaev@profitbricks.com, asahi@lists.linux.dev,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
References: <YvqaK3hxix9AaQBO@slm.duckdns.org>
 <YvsZ6vObgLaDeSZk@gondor.apana.org.au>
 <CAHk-=wgSNiT5qJX53RHtWECsUiFq6d6VWYNAvu71ViOEan07yw@mail.gmail.com>
 <cd51b422-89f3-1856-5d3b-d6e5b0029085@marcan.st>
 <CAHk-=wjfLT7nL8pV8RWATpjgm0zDtUwT8UMtroqnGcXRjN8tgw@mail.gmail.com>
 <24c88c4f-aea5-1fb7-0ead-95c88629d72b@marcan.st>
 <YvtLsNZEUUytdx+i@gondor.apana.org.au>
From:   Hector Martin <marcan@marcan.st>
In-Reply-To: <YvtLsNZEUUytdx+i@gondor.apana.org.au>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 2022/08/16 16:48, Herbert Xu wrote:
> On Tue, Aug 16, 2022 at 03:28:50PM +0900, Hector Martin wrote:
>>
>> This is the same reason I argued queue_work() itself needs to have a
>> similar guarantee, even when it doesn't queue work (and I updated the
>> doc to match). If test_and_set_bit() is used in this kind of context
>> often in the kernel, clearly the current implementation/doc clashes with
>> that.
> 
> Kernel code all over the place rely on the fact that test_and_set_bit
> provides a memory barrier.  So this bug that you've discovered is
> not at all isolated to the workqeueue system.  It'll break the kernel
> in lots of places in exactly the same way.

Now I'm surprised this isn't failing all over the place, given that...
these things are annoyingly subtle.

Still would want Will & Peter to chime in, of course.

>> As I said, I don't have any particular beef in this fight, but this is
>> horribly broken on M1/2 right now, so I'll send a patch to change the
>> bitops instead and you all can fight it out over which way is correct :)
> 
> Please do.

Already did, but I just realized I forgot to Cc you. Sorry about that,
hope you can pick it up through the MLs:

https://lore.kernel.org/asahi/20220816070311.89186-1-marcan@marcan.st/T/#u

- Hector