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SubjectRe: [PATCH] cgroup/cpuset: remove circular dependency deadlock
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@codeaurora.org>
> ---
> 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().

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