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SubjectRe: [PATCH] cgroup/cpuset: remove circular dependency deadlock
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.

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