On 10/30/2017 12:46 PM, 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.
>
> Process A => kthreadd => Process B => Process C => Process A
>
> Process A
> cpu_subsys_offline();
> cpu_down();
> _cpu_down();
> percpu_down_write(&cpu_hotplug_lock); //held
> cpuhp_invoke_callback();
> workqueue_offline_cpu();
> queue_work_on(); // unbind_work on system_highpri_wq
> __queue_work();
> insert_work();
> wake_up_worker();
> flush_work();
> wait_for_completion();
>
> worker_thread();
> manage_workers();
> create_worker();
> kthread_create_on_node();
> wake_up_process(kthreadd_task);
>
> kthreadd
> kthreadd();
> kernel_thread();
> do_fork();
> copy_process();
> percpu_down_read(&cgroup_threadgroup_rwsem);
> __rwsem_down_read_failed_common(); //waiting
>
> Process B
> kernfs_fop_write();
> cgroup_file_write();
> cgroup_procs_write();
> percpu_down_write(&cgroup_threadgroup_rwsem); //held
> cgroup_attach_task();
> cgroup_migrate();
> cgroup_migrate_execute();
> cpuset_can_attach();
> mutex_lock(&cpuset_mutex); //waiting
>
> Process C
> kernfs_fop_write();
> cgroup_file_write();
> cpuset_write_resmask();
> mutex_lock(&cpuset_mutex); //held
> update_cpumask();
> update_cpumasks_hier();
> rebuild_sched_domains_locked();
> get_online_cpus();
> percpu_down_read(&cpu_hotplug_lock); //waiting
>
> Eliminating deadlock by reversing the locking order for cpuset_mutex and
> cpu_hotplug_lock. After inverting the locking sequence of cpu_hotplug_lock
> and cpuset_mutex, cpuset_hotplug_workfn() related functionality can be
> done synchronously from the context doing cpu hotplug. For memory hotplug
> it still gets queued as a work item.
>
> Signed-off-by: Prateek Sood <prsood@xxxxxxxxxxxxxx>
> ---
> include/linux/cpuset.h | 6 ----
> kernel/cgroup/cpuset.c | 94 +++++++++++++++++++++++++++-----------------------
> kernel/power/process.c | 2 --
> kernel/sched/core.c | 1 -
> 4 files changed, 50 insertions(+), 53 deletions(-)
>
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index a1e6a33..e74655d 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -51,9 +51,7 @@ static inline void cpuset_dec(void)
>
> extern int cpuset_init(void);
> extern void cpuset_init_smp(void);
> -extern void cpuset_force_rebuild(void);
> extern void cpuset_update_active_cpus(void);
> -extern void cpuset_wait_for_hotplug(void);
> extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
> extern void cpuset_cpus_allowed_fallback(struct task_struct *p);
> extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
> @@ -166,15 +164,11 @@ static inline void set_mems_allowed(nodemask_t nodemask)
> static inline int cpuset_init(void) { return 0; }
> static inline void cpuset_init_smp(void) {}
>
> -static inline void cpuset_force_rebuild(void) { }
> -
> static inline void cpuset_update_active_cpus(void)
> {
> partition_sched_domains(1, NULL, NULL);
> }
>
> -static inline void cpuset_wait_for_hotplug(void) { }
> -
> static inline void cpuset_cpus_allowed(struct task_struct *p,
> struct cpumask *mask)
> {
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 4657e29..ec44aaa 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -817,6 +817,18 @@ static int generate_sched_domains(cpumask_var_t **domains,
> return ndoms;
> }
>
> +static void cpuset_sched_change_begin(void)
> +{
> + cpus_read_lock();
> + mutex_lock(&cpuset_mutex);
> +}
> +
> +static void cpuset_sched_change_end(void)
> +{
> + mutex_unlock(&cpuset_mutex);
> + cpus_read_unlock();
> +}
> +
> /*
> * Rebuild scheduler domains.
> *
> @@ -826,16 +838,14 @@ 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_held(&cpuset_mutex);
> - get_online_cpus();
>
> /*
> * We have raced with CPU hotplug. Don't do anything to avoid
> @@ -843,27 +853,25 @@ 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)
> {
> - mutex_lock(&cpuset_mutex);
> - rebuild_sched_domains_locked();
> - mutex_unlock(&cpuset_mutex);
> + cpuset_sched_change_begin();
> + rebuild_sched_domains_cpuslocked();
> + cpuset_sched_change_end();
> }
>
> /**
> @@ -949,7 +957,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
> rcu_read_unlock();
>
> if (need_rebuild_sched_domains)
> - rebuild_sched_domains_locked();
> + rebuild_sched_domains_cpuslocked();
> }
>
> /**
> @@ -1281,7 +1289,7 @@ static int update_relax_domain_level(struct cpuset *cs, s64 val)
> cs->relax_domain_level = val;
> if (!cpumask_empty(cs->cpus_allowed) &&
> is_sched_load_balance(cs))
> - rebuild_sched_domains_locked();
> + rebuild_sched_domains_cpuslocked();
> }
>
> return 0;
> @@ -1314,7 +1322,6 @@ static void update_tasks_flags(struct cpuset *cs)
> *
> * Call with cpuset_mutex held.
> */
> -
> static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs,
> int turning_on)
> {
> @@ -1347,7 +1354,7 @@ static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs,
> spin_unlock_irq(&callback_lock);
>
> if (!cpumask_empty(trialcs->cpus_allowed) && balance_flag_changed)
> - rebuild_sched_domains_locked();
> + rebuild_sched_domains_cpuslocked();
>
> if (spread_flag_changed)
> update_tasks_flags(cs);
> @@ -1615,7 +1622,7 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
> cpuset_filetype_t type = cft->private;
> int retval = 0;
>
> - mutex_lock(&cpuset_mutex);
> + cpuset_sched_change_begin();
> if (!is_cpuset_online(cs)) {
> retval = -ENODEV;
> goto out_unlock;
> @@ -1651,7 +1658,7 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
> break;
> }
> out_unlock:
> - mutex_unlock(&cpuset_mutex);
> + cpuset_sched_change_end();
> return retval;
> }
>
> @@ -1662,7 +1669,7 @@ static int cpuset_write_s64(struct cgroup_subsys_state *css, struct cftype *cft,
> cpuset_filetype_t type = cft->private;
> int retval = -ENODEV;
>
> - mutex_lock(&cpuset_mutex);
> + cpuset_sched_change_begin();
> if (!is_cpuset_online(cs))
> goto out_unlock;
>
> @@ -1675,7 +1682,7 @@ static int cpuset_write_s64(struct cgroup_subsys_state *css, struct cftype *cft,
> break;
> }
> out_unlock:
> - mutex_unlock(&cpuset_mutex);
> + cpuset_sched_change_end();
> return retval;
> }
>
> @@ -1714,7 +1721,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
> kernfs_break_active_protection(of->kn);
> flush_work(&cpuset_hotplug_work);
>
> - mutex_lock(&cpuset_mutex);
> + cpuset_sched_change_begin();
> if (!is_cpuset_online(cs))
> goto out_unlock;
>
> @@ -1738,7 +1745,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>
> free_trial_cpuset(trialcs);
> out_unlock:
> - mutex_unlock(&cpuset_mutex);
> + cpuset_sched_change_end();
> kernfs_unbreak_active_protection(of->kn);
> css_put(&cs->css);
> flush_workqueue(cpuset_migrate_mm_wq);
> @@ -2039,14 +2046,14 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
> /*
> * If the cpuset being removed has its flag 'sched_load_balance'
> * enabled, then simulate turning sched_load_balance off, which
> - * will call rebuild_sched_domains_locked().
> + * will call rebuild_sched_domains_cpuslocked().
> */
>
> static void cpuset_css_offline(struct cgroup_subsys_state *css)
> {
> struct cpuset *cs = css_cs(css);
>
> - mutex_lock(&cpuset_mutex);
> + cpuset_sched_change_begin();
>
> if (is_sched_load_balance(cs))
> update_flag(CS_SCHED_LOAD_BALANCE, cs, 0);
> @@ -2054,7 +2061,7 @@ static void cpuset_css_offline(struct cgroup_subsys_state *css)
> cpuset_dec();
> clear_bit(CS_ONLINE, &cs->flags);
>
> - mutex_unlock(&cpuset_mutex);
> + cpuset_sched_change_end();
> }
>
> static void cpuset_css_free(struct cgroup_subsys_state *css)
> @@ -2275,15 +2282,8 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs)
> mutex_unlock(&cpuset_mutex);
> }
>
> -static bool force_rebuild;
> -
> -void cpuset_force_rebuild(void)
> -{
> - force_rebuild = true;
> -}
> -
> /**
> - * cpuset_hotplug_workfn - handle CPU/memory hotunplug for a cpuset
> + * cpuset_hotplug - handle CPU/memory hotunplug for a cpuset
> *
> * This function is called after either CPU or memory configuration has
> * changed and updates cpuset accordingly. The top_cpuset is always
> @@ -2298,7 +2298,7 @@ void cpuset_force_rebuild(void)
> * Note that CPU offlining during suspend is ignored. We don't modify
> * cpusets across suspend/resume cycles at all.
> */
> -static void cpuset_hotplug_workfn(struct work_struct *work)
> +static void cpuset_hotplug(bool use_cpu_hp_lock)
> {
> static cpumask_t new_cpus;
> static nodemask_t new_mems;
> @@ -2356,25 +2356,31 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
> }
>
> /* rebuild sched domains if cpus_allowed has changed */
> - if (cpus_updated || force_rebuild) {
> - force_rebuild = false;
> - rebuild_sched_domains();
> + if (cpus_updated) {
> + if (use_cpu_hp_lock)
> + rebuild_sched_domains();
> + else {
> + /* Acquiring cpu_hotplug_lock is not required.
> + * When cpuset_hotplug() is called in hotplug path,
> + * cpu_hotplug_lock is held by the hotplug context
> + * which is waiting for cpuhp_thread_fun to indicate
> + * completion of callback.
> + */
> + mutex_lock(&cpuset_mutex);
> + rebuild_sched_domains_cpuslocked();
> + mutex_unlock(&cpuset_mutex);
> + }
> }
> }
>
> -void cpuset_update_active_cpus(void)
> +static void cpuset_hotplug_workfn(struct work_struct *work)
> {
> - /*
> - * We're inside cpu hotplug critical region which usually nests
> - * inside cgroup synchronization. Bounce actual hotplug processing
> - * to a work item to avoid reverse locking order.
> - */
> - schedule_work(&cpuset_hotplug_work);
> + cpuset_hotplug(true);
> }
>
> -void cpuset_wait_for_hotplug(void)
> +void cpuset_update_active_cpus(void)
> {
> - flush_work(&cpuset_hotplug_work);
> + cpuset_hotplug(false);
> }
>
> /*
> diff --git a/kernel/power/process.c b/kernel/power/process.c
> index 50f25cb..28772b405 100644
> --- a/kernel/power/process.c
> +++ b/kernel/power/process.c
> @@ -203,8 +203,6 @@ void thaw_processes(void)
> __usermodehelper_set_disable_depth(UMH_FREEZING);
> thaw_workqueues();
>
> - cpuset_wait_for_hotplug();
> -
> read_lock(&tasklist_lock);
> for_each_process_thread(g, p) {
> /* No other threads should have PF_SUSPEND_TASK set */
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d17c5da..25b8717 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5590,7 +5590,6 @@ static void cpuset_cpu_active(void)
> * restore the original sched domains by considering the
> * cpuset configurations.
> */
> - cpuset_force_rebuild();
> }
> cpuset_update_active_cpus();
> }
>
Any improvement/suggestion for this patch?
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