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From:   Qian Cai <cai@lca.pw>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH -next] power/qos: fix a data race in pm_qos_*_value
Date:   Sun, 23 Feb 2020 20:01:33 -0500
Message-Id: <62491094-D13B-4EED-8190-4AA4EB77036B@lca.pw>
References: <CAJZ5v0iSEV9S=zTa9++vUCO6GTfBE2sxNY+b4mMMt4Y6RCRvjA@mail.gmail.com>
Cc:     "Rafael J. Wysocki" <rjw@rjwysocki.net>, elver@google.com,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
In-Reply-To: <CAJZ5v0iSEV9S=zTa9++vUCO6GTfBE2sxNY+b4mMMt4Y6RCRvjA@mail.gmail.com>
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Feb 23, 2020, at 7:12 PM, Rafael J. Wysocki <rafael@kernel.org> wrote:
>=20
> It may be a bug under certain conditions, but you don't mention what
> conditions they are.  Reporting it as a general bug is not accurate at
> the very least.

Could we rule out load tearing, store tearing and reload of global_req in cp=
uidle_governor_latency() for all compilers and architectures which could int=
roduce logic bugs?

	int global_req =3D cpu_latency_qos_limit();

	if (device_req > global_req)
		device_req =3D global_req;

If under register pressure, the compiler might get ride of the tmp variable,=
 i.e.,

If (device_req > cpu_latency_qos_limit())
=E2=80=94-> race with the writer.
         device_req =3D cpu_latency_qos_limit();