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

[Peter Zijlstra <peterz@infradead.org>]

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.