From: Hector Martin <marcan@marcan.st>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>,
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
Subject: Re: [PATCH] workqueue: Fix memory ordering race in queue_work*()
Date: Tue, 16 Aug 2022 15:28:50 +0900 [thread overview]
Message-ID: <24c88c4f-aea5-1fb7-0ead-95c88629d72b@marcan.st> (raw)
In-Reply-To: <CAHk-=wjfLT7nL8pV8RWATpjgm0zDtUwT8UMtroqnGcXRjN8tgw@mail.gmail.com>
On 2022/08/16 14:52, Linus Torvalds wrote:
> I think I understand *why* it's broken - it looks like a "harmless"
> optimization. After all, if the bitop doesn't do anything, there's
> nothing to order it with.
>
> It makes a certain amount of sense - as long as you don't think about
> it too hard.
>
> The reason it is completely and utterly broken is that it's not
> actually just "the bitop doesn't do anything". Even when it doesn't
> change the bit value, just the ordering of the read of the old bit
> value can be meaningful, exactly for that case of "I added more work
> to the queue, I need to set the bit to tell the consumers, and if I'm
> the first person to set the bit I may need to wake the consumer up".
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.
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 :)
--
Hector Martin (marcan@marcan.st)
Public Key: https://mrcn.st/pub
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next prev parent reply other threads:[~2022-08-16 6:30 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-15 17:58 [PATCH] workqueue: Fix memory ordering race in queue_work*() Hector Martin
2022-08-15 19:10 ` Tejun Heo
2022-08-16 4:15 ` Herbert Xu
2022-08-16 5:27 ` Linus Torvalds
2022-08-16 5:36 ` Hector Martin
2022-08-16 5:52 ` Linus Torvalds
2022-08-16 6:28 ` Hector Martin [this message]
2022-08-16 7:48 ` Herbert Xu
2022-08-16 8:01 ` Hector Martin
2022-08-16 5:48 ` Herbert Xu
2022-08-16 6:03 ` Linus Torvalds
2022-08-16 13:41 ` Will Deacon
2022-08-16 14:55 ` Boqun Feng
2022-08-16 16:22 ` Hector Martin
2022-08-16 16:43 ` Boqun Feng
2022-08-16 16:58 ` Linus Torvalds
2022-08-16 16:41 ` Linus Torvalds
2022-08-17 5:05 ` Herbert Xu
2022-08-16 17:01 ` Tejun Heo
2022-08-16 16:26 ` Tejun Heo
2022-08-16 17:21 ` Hector Martin
2022-08-16 4:14 ` Herbert Xu
2022-08-16 5:37 ` Hector Martin
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