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Date:   Sun, 7 Feb 2021 00:25:08 +0100
From:   Pavel Machek <pavel@ucw.cz>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Sasha Levin <sashal@kernel.org>
Subject: Re: [PATCH 4.19 16/17] kthread: Extract KTHREAD_IS_PER_CPU
Message-ID: <20210206232508.GA27515@amd>
References: <20210205140649.825180779@linuxfoundation.org>
 <20210205140650.464297049@linuxfoundation.org>
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Hi!

> From: Peter Zijlstra <peterz@infradead.org>
>=20
> [ Upstream commit ac687e6e8c26181a33270efd1a2e2241377924b0 ]
>=20
> There is a need to distinguish geniune per-cpu kthreads from kthreads
> that happen to have a single CPU affinity.
>=20
> Geniune per-cpu kthreads are kthreads that are CPU affine for
> correctness, these will obviously have PF_KTHREAD set, but must also
> have PF_NO_SETAFFINITY set, lest userspace modify their affinity and
> ruins things.
>=20
> However, these two things are not sufficient, PF_NO_SETAFFINITY is
> also set on other tasks that have their affinities controlled through
> other means, like for instance workqueues.
>=20
> Therefore another bit is needed; it turns out kthread_create_per_cpu()
> already has such a bit: KTHREAD_IS_PER_CPU, which is used to make
> kthread_park()/kthread_unpark() work correctly.
>=20
> Expose this flag and remove the implicit setting of it from
> kthread_create_on_cpu(); the io_uring usage of it seems dubious at
> best.

AFAIK this should not be in 4.19/5.10 as it does not fix anything w/o
5ba2ffba13a1e. Nobody calls kthread_is_per_cpu() in those kernels.

Best regards,
								Pavel

--=20
http://www.livejournal.com/~pavelmachek

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