linux-kernel - Re: [PATCH -next] power/qos: fix a data race in pm_qos_*

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Message-Id: <62491094-D13B-4EED-8190-4AA4EB77036B@lca.pw>
Date:   Sun, 23 Feb 2020 20:01:33 -0500
From:   Qian Cai <cai@....pw>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>, elver@...gle.com,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] power/qos: fix a data race in pm_qos_*_value



> On Feb 23, 2020, at 7:12 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> 
> 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 cpuidle_governor_latency() for all compilers and architectures which could introduce logic bugs?

	int global_req = cpu_latency_qos_limit();

	if (device_req > global_req)
		device_req = global_req;

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

If (device_req > cpu_latency_qos_limit())
—-> race with the writer.
         device_req = cpu_latency_qos_limit();