linux-kernel - Re: [PATCH] workqueue: Fix memory ordering race in queue

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Date:   Tue, 16 Aug 2022 14:37:33 +0900
From:   Hector Martin <marcan@...can.st>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     tj@...nel.org, will@...nel.org, peterz@...radead.org,
        jirislaby@...nel.org, maz@...nel.org, mark.rutland@....com,
        boqun.feng@...il.com, catalin.marinas@....com, oneukum@...e.com,
        roman.penyaev@...fitbricks.com, asahi@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] workqueue: Fix memory ordering race in queue_work*()

On 16/08/2022 13.14, Herbert Xu wrote:
> Hector Martin <marcan@...can.st> wrote:
>>
>> This has been broken since the dawn of time, and it was incompletely
>> fixed by 346c09f80459, which added the necessary barriers in the work
>> execution path but failed to account for the missing barrier in the
>> test_and_set_bit() failure case. Fix it by switching to
>> atomic_long_fetch_or(), which does have unconditional barrier semantics
>> regardless of whether the bit was already set or not (this is actually
>> just test_and_set_bit() minus the early exit path).
> 
> test_and_set_bit is supposed to contain a full memory barrier.
> If it doesn't then your arch is broken and needs to be fixed.
> 
> Changing this one spot is pointless because such assumptions
> are all over the kernel.

Documentation/atomic_bitops.txt and the asm-generic implementaton
disagree with you, so this isn't quite as simple as "your arch is
broken" :-)

- Hector