Re: [PATCH] cgroup/cpuset: remove circular dependency deadlock

[Prateek Sood <prsood@codeaurora.org>]

On 09/07/2017 12:58 PM, Peter Zijlstra wrote:
> On Thu, Sep 07, 2017 at 11:34:12AM +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.
>>
>> Example scenario:
>> kworker/0:0 => kthreadd => init:729 => init:1 => kworker/0:0
>>
>> kworker/0:0 - percpu_down_write(&cpu_hotplug_lock)  [held]
>>               flush(work)   [no high prio workqueue available on CPU]
>>               wait_for_completion()
>>
>> kthreadd    - percpu_down_read(cgroup_threadgroup_rwsem)  [waiting]
>>
>> init:729    - percpu_down_write(cgroup_threadgroup_rwsem)   [held]
>>               lock(cpuset_mutex)   [waiting]
>>
>> init:1      - lock(cpuset_mutex)   [held]
>>               percpu_down_read(&cpu_hotplug_lock)   [waiting]
> 
> That's both unreadable and useless :/ You want to tell what code paths
> that were, not which random tasks happened to run them.
> 
> 
> 
>> Eliminate this dependecy by reordering locking of cpuset_mutex
>> and cpu_hotplug_lock in following order
>> 1. Acquire cpu_hotplug_lock (read)
>> 2. Acquire cpuset_mutex
>>
>> Signed-off-by: Prateek Sood <prsood@codeaurora.org>
>> ---
>>  kernel/cgroup/cpuset.c | 70 +++++++++++++++++++++++++++++++++++++++++++-------
>>  1 file changed, 61 insertions(+), 9 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 2f4039b..687be57 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -843,10 +843,41 @@ static void rebuild_sched_domains_locked(void)
>>  out:
>>  	put_online_cpus();
>>  }
>> +
>> +/*
>> + * Rebuild scheduler domains.
>> + * Call with following lock held in the order
>> + * 1. cpu_hotplug_lock (read)
>> + * 2. cpuset_mutex
> 
> Do not put that in comments, nobody ever reads comments.
> 
>> + */
>> +static void rebuild_sched_domains_unlocked(void)
> 
> The common postfix for a function called with the cpuhotplug lock held
> is: _cpuslocked()
> 
>> +{
>> +        struct sched_domain_attr *attr;
>> +        cpumask_var_t *doms;
>> +        int ndoms;
> 
> 	lockdep_assert_cpus_held();
> 	lockdep_assert_held(&cpuset_mutex);
> 
>> +
>> +        /*
>> +         * We have raced with CPU hotplug. Don't do anything to avoid
>> +         * passing doms with offlined cpu to partition_sched_domains().
>> +         * Anyways, hotplug work item will rebuild sched domains.
>> +         */
>> +        if (!cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
>> +                return;
>> +
>> +        /* Generate domain masks and attrs */
>> +        ndoms = generate_sched_domains(&doms, &attr);
>> +
>> +        /* Have scheduler rebuild the domains */
>> +        partition_sched_domains(ndoms, doms, attr);
>> +}
> 
> And you couldn't come up with a way to share _anything_ with the
> existing rebuild_sched_domains_locked() function?
> 
> *sigh*.. please try again.
> 

Thanks for the suggestion Peter, I will resend the patch

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