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SubjectRe: [PATCH 2/2] cpuhotplug: introduce try_get_online_cpus()
Andrew Morton wrote:
>
>> ---
>> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
>> index 2643d84..98f5c4b 100644
>> --- a/include/linux/cpu.h
>> +++ b/include/linux/cpu.h
>> @@ -104,6 +104,7 @@ extern struct sysdev_class cpu_sysdev_class;
>>
>> extern void get_online_cpus(void);
>> extern void put_online_cpus(void);
>> +extern int try_get_online_cpus(void);
>> #define hotcpu_notifier(fn, pri) { \
>> static struct notifier_block fn##_nb __cpuinitdata = \
>> { .notifier_call = fn, .priority = pri }; \
>> @@ -117,6 +118,7 @@ int cpu_down(unsigned int cpu);
>>
>> #define get_online_cpus() do { } while (0)
>> #define put_online_cpus() do { } while (0)
>> +#define try_get_online_cpus() (1)
>> #define hotcpu_notifier(fn, pri) do { (void)(fn); } while (0)
>> /* These aren't inline functions due to a GCC bug. */
>> #define register_hotcpu_notifier(nb) ({ (void)(nb); 0; })
>> diff --git a/kernel/cpu.c b/kernel/cpu.c
>> index 62198ec..e948f19 100644
>> --- a/kernel/cpu.c
>> +++ b/kernel/cpu.c
>> @@ -66,6 +66,15 @@ void put_online_cpus(void)
>> }
>> EXPORT_SYMBOL_GPL(put_online_cpus);
>>
>> +int try_get_online_cpus(void)
>> +{
>> + might_sleep();
>> + if (cpu_hotplug.active_writer == current)
>> + return 1;
>> + return down_read_trylock(&cpu_hotplug.rwlock);
>> +
>> +}
>> +EXPORT_SYMBOL_GPL(try_get_online_cpus);
>
> It's strange to add a might_sleep() to a function which doesn't sleep.

It might sleep indeed. I prefer to add a might_sleep()/cond_sched()
for a sleepable function.

>
> The patch adds no callers to this function. This is significant
> because it would be quite interesting to find out which subsystem(s)
> you've found to have this deadlock. I do think that we should look at
> alternative (non-trylocky) ways of fixing them.
>

This problem exist in cgroup/cpuset. and Max Krasnyansky fix it:
(non-trylocky way)
*
* The rebuild_sched_domains() and partition_sched_domains()
* routines must nest cgroup_lock() inside get_online_cpus(),
* but such cpuset changes as these must nest that locking the
* other way, holding cgroup_lock() for much of the code.
*
* So in order to avoid an ABBA deadlock, the cpuset code handling
* these user changes delegates the actual sched domain rebuilding
* to a separate workqueue thread, which ends up processing the
* above do_rebuild_sched_domains() function.
*/
static void async_rebuild_sched_domains(void)
{
queue_work(cpuset_wq, &rebuild_sched_domains_work);
}

get_online_cpus() is so a coarsely granular lock, and
try_get_online_cpus() fails rarely. The kernel indeed needs
try_get_online_cpus().

Paul will use it:
http://lkml.org/lkml/2009/5/22/332

Lai




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