On 06/03/2015 03:57 PM, Viresh Kumar wrote:
There are several races reported in cpufreq core around governors (only ondemand and conservative) by different people.
There are at least two race scenarios present in governor code: (a) Concurrent access/updates of governor internal structures.
It is possible that fields such as 'dbs_data->usage_count', etc. are accessed simultaneously for different policies using same governor structure (i.e. CPUFREQ_HAVE_GOVERNOR_PER_POLICY flag unset). And because of this we can dereference bad pointers.
For example consider a system with two CPUs with separate 'struct cpufreq_policy' instances. CPU0 governor: ondemand and CPU1: powersave. CPU0 switching to powersave and CPU1 to ondemand: CPU0 CPU1
store* store*
cpufreq_governor_exit() cpufreq_governor_init() dbs_data = cdata->gdbs_data;
if (!--dbs_data->usage_count) kfree(dbs_data);
dbs_data->usage_count++; *Bad pointer dereference*There are other races possible between EXIT and START/STOP/LIMIT as well. Its really complicated.
(b) Switching governor state in bad sequence:
For example trying to switch a governor to START state, when the governor is in EXIT state. There are some checks present in __cpufreq_governor() but they aren't sufficient as they compare events against 'policy->governor_enabled', where as we need to take governor's state into account, which can be used by multiple policies.
These two issues need to be solved separately and the responsibility should be properly divided between cpufreq and governor core.
The first problem is more about the governor core, as it needs to protect its structures properly. And the second problem should be fixed in cpufreq core instead of governor, as its all about sequence of events.
This patch is trying to solve only the first problem.
There are two types of data we need to protect,
- 'struct common_dbs_data': No matter what, there is going to be a single copy of this per governor.
- 'struct dbs_data': With CPUFREQ_HAVE_GOVERNOR_PER_POLICY flag set, we will have per-policy copy of this data, otherwise a single copy.
Because of such complexities, the mutex present in 'struct dbs_data' is insufficient to solve our problem. For example we need to protect fetching of 'dbs_data' from different structures at the beginning of cpufreq_governor_dbs(), to make sure it isn't currently being updated.
This can be fixed if we can guarantee serialization of event parsing code for an individual governor. This is best solved with a mutex per governor, and the placeholder for that is 'struct common_dbs_data'.
And so this patch moves the mutex from 'struct dbs_data' to 'struct common_dbs_data' and takes it at the beginning and drops it at the end of cpufreq_governor_dbs().
Tested with and without following configuration options:
CONFIG_LOCKDEP_SUPPORT=y CONFIG_DEBUG_RT_MUTEXES=y CONFIG_DEBUG_PI_LIST=y CONFIG_DEBUG_SPINLOCK=y CONFIG_DEBUG_MUTEXES=y CONFIG_DEBUG_LOCK_ALLOC=y CONFIG_PROVE_LOCKING=y CONFIG_LOCKDEP=y CONFIG_DEBUG_ATOMIC_SLEEP=y
Signed-off-by: Viresh Kumar viresh.kumar@linaro.org
Reviewed-by: Preeti U Murthy preeti@linux.vnet.ibm.com