Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Mon, 24 Feb 2020 10:54:33 +0100 | Subject | Re: [PATCH -next] power/qos: fix a data race in pm_qos_*_value |
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On Mon, Feb 24, 2020 at 2:01 AM Qian Cai <cai@lca.pw> wrote: > > > > > On Feb 23, 2020, at 7:12 PM, Rafael J. Wysocki <rafael@kernel.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();
Yes, there is a race here with or without the WRITE_ONCE()/READ_ONCE() annotations (note that these annotations don't prevent CPUs from reordering things, so device_req may be set before global_req regardless).
However, worst-case it may cause an old value to be used and that can happen anyway if the entire cpuidle_governor_latency_req() runs between the curr_value update and pm_qos_set_value() in pm_qos_update_target(), for example.
IOW, there is no guarantee that the new value will be used immediately after updating a QoS request anyway.
I agree with adding the annotations (I was considering posting a patch doing that myself), but just as a matter of making the intention clear.
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