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devfreq: memlat: fix potential deadlock during CPU hotplug

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The cpu_grp->mons_lock is acquired in the memlat hotplug notifier
callbacks to prevent races within the driver. The cpus_read_lock is
inherently acquired as well since it is in the middle of a hotplug
event. In parallel, stop_hwmon() could unregister from the hotplug
notifier (i.e. when last memlat mon in system is removed). However,
unregistering the hotplug notifier also requires the cpus_read_lock
and is done with the cpu_grp->mons_lock held as well which would lead
to a deadlock if a hotplug is currently in progress with the memlat
notifier callback waiting for the cpu_grp->mons_lock.

To fix this, release the cpu_grp->mons_lock in stop_hwmon() before
unregistering with the hotplug notifier. Also, make sure the memlat
hotplug notifier callbacks check cpu_grp->num_active_mons before
restarting events since that work won't be necessary if there are no
active memlat mons in the system.

Change-Id: I401639a6e28a472a64a96456d2c06f57b35dbc0d
Signed-off-by: Amir Vajid <avajid@codeaurora.org>
This commit is contained in:
Amir Vajid 2020-06-09 16:41:42 -07:00
parent f5090438b6
commit d0142cbf68
1 changed files with 9 additions and 5 deletions
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14
drivers/devfreq/arm-memlat-mon.c
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@ -461,8 +461,10 @@ static int memlat_event_hotplug_coming_up(unsigned int cpu)
if (!cpu_grp)
return -ENODEV;
mutex_lock(&cpu_grp->mons_lock);
ret = memlat_hp_restart_events(cpu, true);
per_cpu(cpu_is_hp, cpu) = false;
if (cpu_grp->num_active_mons) {
ret = memlat_hp_restart_events(cpu, true);
per_cpu(cpu_is_hp, cpu) = false;
}
mutex_unlock(&cpu_grp->mons_lock);
return ret;
@ -477,8 +479,10 @@ static int memlat_event_hotplug_going_down(unsigned int cpu)
return -ENODEV;
/* avoid race between cpu hotplug and update_counts */
mutex_lock(&cpu_grp->mons_lock);
per_cpu(cpu_is_hp, cpu) = true;
ret = memlat_hp_restart_events(cpu, false);
if (cpu_grp->num_active_mons) {
per_cpu(cpu_is_hp, cpu) = true;
ret = memlat_hp_restart_events(cpu, false);
}
mutex_unlock(&cpu_grp->mons_lock);
return ret;
@ -629,6 +633,7 @@ static void stop_hwmon(struct memlat_hwmon *hw)
hp_idle_register_cnt--;
mutex_unlock(&notify_lock);
}
mutex_unlock(&cpu_grp->mons_lock);
mutex_lock(&notify_lock);
if (!hp_idle_register_cnt) {
cpuhp_remove_state_nocalls(CPUHP_AP_ONLINE_DYN);
@ -639,7 +644,6 @@ static void stop_hwmon(struct memlat_hwmon *hw)
}
}
mutex_unlock(&notify_lock);
mutex_unlock(&cpu_grp->mons_lock);
}
/**