[PATCH] cpufreq: governor: Fix race between sysfs store and dbs work handler

[Zhongqiu Han]

gov_update_cpu_data() resets per-CPU prev_cpu_idle and prev_cpu_nice
for every CPU in the governed domain. It is called from sysfs store
callbacks (e.g. ignore_nice_load_store) which run under
attr_set->update_lock, held by the surrounding governor_store().

Concurrently, dbs_work_handler() calls gov->gov_dbs_update() (which
calls dbs_update()) under policy_dbs->update_mutex. dbs_update() both
reads and writes the same prev_cpu_idle / prev_cpu_nice fields. The
potential race path is:

Path A (sysfs write, holds attr_set->update_lock only):

  governor_store()
    mutex_lock(&attr_set->update_lock)
    ignore_nice_load_store()
      dbs_data->ignore_nice_load = input
      gov_update_cpu_data(dbs_data)
        list_for_each_entry(policy_dbs, ...)
          for_each_cpu(j, ...)
            j_cdbs->prev_cpu_idle = get_cpu_idle_time(...)  /* write */
            j_cdbs->prev_cpu_nice = kcpustat_field(...)     /* write */
    mutex_unlock(&attr_set->update_lock)

Path B (work queue, holds policy_dbs->update_mutex only):

  dbs_work_handler()
    mutex_lock(&policy_dbs->update_mutex)
    gov->gov_dbs_update(policy)
      dbs_update()
        for_each_cpu(j, policy->cpus)
          idle_time = cur - j_cdbs->prev_cpu_idle           /* read  */
          j_cdbs->prev_cpu_idle = cur_idle_time             /* write */
          idle_time += cur_nice - j_cdbs->prev_cpu_nice     /* read  */
          j_cdbs->prev_cpu_nice = cur_nice                  /* write */
    mutex_unlock(&policy_dbs->update_mutex)

Because attr_set->update_lock and policy_dbs->update_mutex are two
completely independent locks, the two paths are not mutually exclusive.
This results in a data race on cpu_dbs_info.prev_cpu_idle and
cpu_dbs_info.prev_cpu_nice.

Fix this by also acquiring policy_dbs->update_mutex in
gov_update_cpu_data() for each policy, so that path A participates in
the mutual exclusion already established by dbs_work_handler(). Also
update the function comment to accurately reflect the two-level locking
contract.

The root of this race dates back to the original ondemand/conservative
governors. Before commit ee88415caf73 ("[CPUFREQ] Cleanup locking in
conservative governor") and commit 5a75c82828e7 ("[CPUFREQ] Cleanup
locking in ondemand governor"), all accesses to prev_cpu_idle and
prev_cpu_nice in cpufreq_governor_dbs() (path X), store_ignore_nice_load()
(path Y), and do_dbs_timer() (path Z) were serialised by the same
dbs_mutex, so no race existed. Those two commits switched do_dbs_timer()
from dbs_mutex to a per-policy/per-cpu timer_mutex to reduce lock
contention, but left store_ignore_nice_load() still holding dbs_mutex.
As a result, path Y (store) and path Z (do_dbs_timer) no longer shared a
common lock, introducing a potential race on prev_cpu_idle/prev_cpu_nice
between store_ignore_nice_load() and dbs_check_cpu().

Commit 326c86deaed54a ("[CPUFREQ] Remove unneeded locks") then removed
dbs_mutex from store_ignore_nice_load() entirely, introducing an
additional potential race between store_ignore_nice_load() (path Y, now
lockless) and cpufreq_governor_dbs() (path X, still holding dbs_mutex),
while the race between path Y and path Z remained.

Fixes: ee88415caf736b ("[CPUFREQ] Cleanup locking in conservative governor")
Fixes: 5a75c82828e7c0 ("[CPUFREQ] Cleanup locking in ondemand governor")
Fixes: 326c86deaed54a ("[CPUFREQ] Remove unneeded locks")
Signed-off-by: Zhongqiu Han <zhongqiu.han@xxxxxxxxxxxxxxxx>
---
 drivers/cpufreq/cpufreq_governor.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index 86f35e451914..56ef793362db 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c
@@ -90,7 +90,8 @@ EXPORT_SYMBOL_GPL(sampling_rate_store);
  * (that may be a single policy or a bunch of them if governor tunables are
  * system-wide).
  *
- * Call under the @dbs_data mutex.
+ * Call under the @dbs_data->attr_set.update_lock. The per-policy
+ * update_mutex is acquired and released internally for each policy.
  */
 void gov_update_cpu_data(struct dbs_data *dbs_data)
 {
@@ -99,6 +100,7 @@ void gov_update_cpu_data(struct dbs_data *dbs_data)
 	list_for_each_entry(policy_dbs, &dbs_data->attr_set.policy_list, list) {
 		unsigned int j;
 
+		mutex_lock(&policy_dbs->update_mutex);
 		for_each_cpu(j, policy_dbs->policy->cpus) {
 			struct cpu_dbs_info *j_cdbs = &per_cpu(cpu_dbs, j);
 
@@ -107,6 +109,7 @@ void gov_update_cpu_data(struct dbs_data *dbs_data)
 			if (dbs_data->ignore_nice_load)
 				j_cdbs->prev_cpu_nice = kcpustat_field(&kcpustat_cpu(j), CPUTIME_NICE, j);
 		}
+		mutex_unlock(&policy_dbs->update_mutex);
 	}
 }
 EXPORT_SYMBOL_GPL(gov_update_cpu_data);
-- 
2.43.0