Messages in this thread | | | Date | Tue, 29 Aug 2023 14:17:30 +0800 | Subject | Re: [PATCH] cpufreq: Fix the race condition while updating the transition_task of policy | From | "Liao, Chang" <> |
| |
Hi, Rafael
在 2023/8/28 16:58, Rafael J. Wysocki 写道: > On Mon, Aug 28, 2023 at 10:52 AM Viresh Kumar <viresh.kumar@linaro.org> wrote: >> >> On 28-08-23, 16:29, Liao, Chang wrote: >>> Task B does not necessarily go to sleep when it calls wait_event(), it depends on >>> the condition to wait for evaluate false or not. So there is a small race window >>> where Task A already set 'transition_ongoing' to false and Task B can cross wait_event() >>> immediately. >>> >>> wait_event: >>> do { >>> might_sleep(); >>> if (condition) // !transition_ongoing >>> break; >>> __wait_event(); >>> }; >>> >>> I hope I do not miss something important in the code above. >> >>> Yes, if the CPU uses weak memroy model, it is possible for the instructions to be reordered. >>> therefore, it is a good idea to insert an smb() between these two lines if there is race here. >> >> Maybe it would be better to do this instead ? >> >> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c >> index 6b52ebe5a890..f11b01b25e8d 100644 >> --- a/drivers/cpufreq/cpufreq.c >> +++ b/drivers/cpufreq/cpufreq.c >> @@ -455,8 +455,10 @@ void cpufreq_freq_transition_end(struct cpufreq_policy *policy, >> policy->cur, >> policy->cpuinfo.max_freq); >> >> + spin_lock(&policy->transition_lock); >> policy->transition_ongoing = false; >> policy->transition_task = NULL; >> + spin_unlock(&policy->transition_lock); >> >> wake_up(&policy->transition_wait); >> } >> >> -- > > I was about to suggest the same thing. > > wake_up() is a full memory barrier only if it actually wakes up a task > and if it doesn't do that, without the locking the other task may see > a state in which transition_ongoing is false already and > transition_task is still NULL regardless of the relative ordering of > the statements before the wake_up() call.
I agree, unless the transition_ongoing and transition_task fields are updated atomically, there is always a window where inconsistency can occur in the policy structure.
-- BR Liao, Chang
| |