Messages in this thread | | | Subject | Re: [PATCH -next] power/qos: fix a data race in pm_qos_*_value | From | Qian Cai <> | Date | Mon, 24 Feb 2020 14:02:39 -0500 |
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On Mon, 2020-02-24 at 10:54 +0100, Rafael J. Wysocki wrote: > 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.
OK, how about this updated texts?
[PATCH -next] power/qos: annotate a data race in pm_qos_*_value
cpu_latency_constraints.target_value could be accessed concurrently via,
cpu_latency_qos_apply pm_qos_update_target pm_qos_set_value
cpuidle_governor_latency_req cpu_latency_qos_limit pm_qos_read_value
The read is outside pm_qos_lock critical section which results in a data race. However, the worst case is that an old value to be used and that can happen anyway, so annotate this data race using a pair of READ|WRITE_ONCE().
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