Re: [PATCH] workqueue: Fix memory ordering race in queue_work*()

[Tejun Heo]

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