Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 5 Nov 2020 12:44:08 +0100
From:   Pavel Machek <pavel@ucw.cz>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Nicholas Piggin <npiggin@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Sasha Levin <sashal@kernel.org>
Subject: Re: [PATCH 4.19 057/191] sparc64: remove mm_cpumask clearing to fix
 kthread_use_mm race
Message-ID: <20201105114408.GA9009@duo.ucw.cz>
References: <20201103203232.656475008@linuxfoundation.org>
 <20201103203240.110227839@linuxfoundation.org>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha1;
        protocol="application/pgp-signature"; boundary="W/nzBZO5zC0uMSeA"
Content-Disposition: inline
In-Reply-To: <20201103203240.110227839@linuxfoundation.org>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk


--W/nzBZO5zC0uMSeA
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

Hi!

> From: Nicholas Piggin <npiggin@gmail.com>
>=20
> [ Upstream commit bafb056ce27940c9994ea905336aa8f27b4f7275 ]
=2E..
> io_uring 2b188cc1bb857 ("Add io_uring IO interface") added code which
> does a kthread_use_mm() from a mmget_not_zero() refcount.
=2E..
> The basic fix for sparc64 is to remove its mm_cpumask clearing code. The
> optimisation could be effectively restored by sending IPIs to mm_cpumask
> members and having them remove themselves from mm_cpumask. This is more
> tricky so I leave it as an exercise for someone with a sparc64 SMP.
> powerpc has a (currently similarly broken) example.

So this removes optimalization from Sparc, because it clashes with
2b188cc1bb857 ("Add io_uring IO interface"). But that commit is not
present in 4.19... so this probably is not good idea.

Best regards,
								Pavel
--=20
http://www.livejournal.com/~pavelmachek

--W/nzBZO5zC0uMSeA
Content-Type: application/pgp-signature; name="signature.asc"

-----BEGIN PGP SIGNATURE-----

iF0EABECAB0WIQRPfPO7r0eAhk010v0w5/Bqldv68gUCX6PliAAKCRAw5/Bqldv6
8n98AJ9Uc+NmgNmzCQI3QdHmr+ziaK4ztwCfUlXuJoy1lBQzAM9E15amAF34hYU=
=m0CZ
-----END PGP SIGNATURE-----

--W/nzBZO5zC0uMSeA--