On 09/07/2017 11:21 PM, Peter Zijlstra wrote:
> On Thu, Sep 07, 2017 at 07:26:23PM +0530, Prateek Sood wrote:
>> Remove circular dependency deadlock in a scenario where hotplug of CPU is
>> being done while there is updation in cgroup and cpuset triggered from
>> userspace.
>
> You've forgotten to mention your solution to the deadlock, namely
> inverting cpuset_mutex and cpu_hotplug_lock.
>
>> Signed-off-by: Prateek Sood <prsood@xxxxxxxxxxxxxx>
>> ---
>> kernel/cgroup/cpuset.c | 32 +++++++++++++++++++-------------
>> 1 file changed, 19 insertions(+), 13 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 2f4039b..60dc0ac 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -816,16 +816,15 @@ static int generate_sched_domains(cpumask_var_t **domains,
>> * 'cpus' is removed, then call this routine to rebuild the
>> * scheduler's dynamic sched domains.
>> *
>> - * Call with cpuset_mutex held. Takes get_online_cpus().
>> */
>> -static void rebuild_sched_domains_locked(void)
>> +static void rebuild_sched_domains_cpuslocked(void)
>> {
>> struct sched_domain_attr *attr;
>> cpumask_var_t *doms;
>> int ndoms;
>>
>> + lockdep_assert_cpus_held();
>> lockdep_assert_held(&cpuset_mutex);
>> - get_online_cpus();
>>
>> /*
>> * We have raced with CPU hotplug. Don't do anything to avoid
>> @@ -833,27 +832,27 @@ static void rebuild_sched_domains_locked(void)
>> * Anyways, hotplug work item will rebuild sched domains.
>> */
>> if (!cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
>> - goto out;
>> + return;
>>
>> /* Generate domain masks and attrs */
>> ndoms = generate_sched_domains(&doms, &attr);
>>
>> /* Have scheduler rebuild the domains */
>> partition_sched_domains(ndoms, doms, attr);
>> -out:
>> - put_online_cpus();
>> }
>> #else /* !CONFIG_SMP */
>> -static void rebuild_sched_domains_locked(void)
>> +static void rebuild_sched_domains_cpuslocked(void)
>> {
>> }
>> #endif /* CONFIG_SMP */
>>
>> void rebuild_sched_domains(void)
>> {
>> + get_online_cpus();
>> mutex_lock(&cpuset_mutex);
>> - rebuild_sched_domains_locked();
>> + rebuild_sched_domains_cpuslocked();
>> mutex_unlock(&cpuset_mutex);
>> + put_online_cpus();
>> }
>
> But if you invert these locks, the need for cpuset_hotplug_workfn() goes
> away, at least for the CPU part, and we can make in synchronous again.
> Yay!!
>
> Also, I think new code should use cpus_read_lock() instead of
> get_online_cpus().
>
Thanks for the review comments Peter.
For patch related to circular deadlock, I will send an updated version.
The callback making a call to cpuset_hotplug_workfn()in hotplug path are
[CPUHP_AP_ACTIVE] = {
.name = "sched:active",
.startup.single = sched_cpu_activate,
.teardown.single = sched_cpu_deactivate,
},
if we make cpuset_hotplug_workfn() synchronous, deadlock might happen:
_cpu_down()
cpus_write_lock() //held
cpuhp_kick_ap_work()
cpuhp_kick_ap()
__cpuhp_kick_ap()
wake_up_process() //cpuhp_thread_fun
wait_for_ap_thread() //wait for complete from cpuhp_thread_fun()
cpuhp_thread_fun()
cpuhp_invoke_callback()
sched_cpu_deactivate()
cpuset_cpu_inactive()
cpuset_update_active_cpus()
cpuset_hotplug_work()
rebuild_sched_domains()
cpus_read_lock() //waiting as acquired in _cpu_down()
--
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