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

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Message-ID: <YvqaK3hxix9AaQBO@slm.duckdns.org>
Date:   Mon, 15 Aug 2022 09:10:35 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Hector Martin <marcan@...can.st>
Cc:     Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, jirislaby@...nel.org,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Oliver Neukum <oneukum@...e.com>,
        Roman Pen <roman.penyaev@...fitbricks.com>,
        Asahi Linux <asahi@...ts.linux.dev>,
        Linux ARM <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 02:58:10AM +0900, Hector Martin 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).
...

Oh, tricky one and yeah you're absolutely right that it makes no sense to
not guarantee barrier semantics when already pending. I didn't even know
test_and_set_bit() wasn't a barrier when it failed. Thanks a lot for hunting
down and fixing this. Applied to wq/for-6.0-fixes.

Thanks.

-- 
tejun