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

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Message-ID: <YvtLsNZEUUytdx+i@gondor.apana.org.au>
Date:   Tue, 16 Aug 2022 15:48:00 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Hector Martin <marcan@...can.st>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will@...nel.org>, Tejun Heo <tj@...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 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.

> 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.

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt