Re: [PATCH v8 6/8] cgroup/cpuset: Change cpuset_rwsem and hotplug lock order

[Peter Zijlstra]

On Fri, Jun 28, 2019 at 10:06:16AM +0200, Juri Lelli wrote:
> cpuset_rwsem is going to be acquired from sched_setscheduler() with a
> following patch. There are however paths (e.g., spawn_ksoftirqd) in
> which sched_scheduler() is eventually called while holding hotplug lock;
> this creates a dependecy between hotplug lock (to be always acquired
> first) and cpuset_rwsem (to be always acquired after hotplug lock).
> 
> Fix paths which currently take the two locks in the wrong order (after
> a following patch is applied).
> Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>

This all reminds me of this:

  https://lkml.kernel.org/r/1510755615-25906-1-git-send-email-prsood@xxxxxxxxxxxxxx

Which sadly got reverted again. If we do this now (I've always been a
proponent), then we can make that rebuild synchronous again, which
should also help here IIRC.

> ---
>  include/linux/cpuset.h |  8 ++++----
>  kernel/cgroup/cpuset.c | 22 +++++++++++++++++-----
>  2 files changed, 21 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index 934633a05d20..7f1478c26a33 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -40,14 +40,14 @@ static inline bool cpusets_enabled(void)
>  
>  static inline void cpuset_inc(void)
>  {
> -	static_branch_inc(&cpusets_pre_enable_key);
> -	static_branch_inc(&cpusets_enabled_key);
> +	static_branch_inc_cpuslocked(&cpusets_pre_enable_key);
> +	static_branch_inc_cpuslocked(&cpusets_enabled_key);
>  }
>  
>  static inline void cpuset_dec(void)
>  {
> -	static_branch_dec(&cpusets_enabled_key);
> -	static_branch_dec(&cpusets_pre_enable_key);
> +	static_branch_dec_cpuslocked(&cpusets_enabled_key);
> +	static_branch_dec_cpuslocked(&cpusets_pre_enable_key);
>  }
>  
>  extern int cpuset_init(void);
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index a7c0c8d8f132..d92b351f89e3 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1026,8 +1026,8 @@ static void rebuild_sched_domains_locked(void)
>  	cpumask_var_t *doms;
>  	int ndoms;
>  
> +	lockdep_assert_cpus_held();
>  	percpu_rwsem_assert_held(&cpuset_rwsem);
> -	get_online_cpus();
>  
>  	/*
>  	 * We have raced with CPU hotplug. Don't do anything to avoid
> @@ -1036,19 +1036,17 @@ static void rebuild_sched_domains_locked(void)
>  	 */
>  	if (!top_cpuset.nr_subparts_cpus &&
>  	    !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
> -		goto out;
> +		return;
>  
>  	if (top_cpuset.nr_subparts_cpus &&
>  	   !cpumask_subset(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_and_rebuild_sched_domains(ndoms, doms, attr);
> -out:
> -	put_online_cpus();
>  }
>  #else /* !CONFIG_SMP */
>  static void rebuild_sched_domains_locked(void)
> @@ -1058,9 +1056,11 @@ static void rebuild_sched_domains_locked(void)
>  
>  void rebuild_sched_domains(void)
>  {
> +	get_online_cpus();
>  	percpu_down_write(&cpuset_rwsem);
>  	rebuild_sched_domains_locked();
>  	percpu_up_write(&cpuset_rwsem);
> +	put_online_cpus();
>  }
>  
>  /**
> @@ -2298,6 +2298,7 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
>  	cpuset_filetype_t type = cft->private;
>  	int retval = 0;
>  
> +	get_online_cpus();
>  	percpu_down_write(&cpuset_rwsem);
>  	if (!is_cpuset_online(cs)) {
>  		retval = -ENODEV;
> @@ -2335,6 +2336,7 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
>  	}
>  out_unlock:
>  	percpu_up_write(&cpuset_rwsem);
> +	put_online_cpus();
>  	return retval;
>  }
>  
> @@ -2345,6 +2347,7 @@ static int cpuset_write_s64(struct cgroup_subsys_state *css, struct cftype *cft,
>  	cpuset_filetype_t type = cft->private;
>  	int retval = -ENODEV;
>  
> +	get_online_cpus();
>  	percpu_down_write(&cpuset_rwsem);
>  	if (!is_cpuset_online(cs))
>  		goto out_unlock;
> @@ -2359,6 +2362,7 @@ static int cpuset_write_s64(struct cgroup_subsys_state *css, struct cftype *cft,
>  	}
>  out_unlock:
>  	percpu_up_write(&cpuset_rwsem);
> +	put_online_cpus();
>  	return retval;
>  }
>  
> @@ -2397,6 +2401,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>  	kernfs_break_active_protection(of->kn);
>  	flush_work(&cpuset_hotplug_work);
>  
> +	get_online_cpus();
>  	percpu_down_write(&cpuset_rwsem);
>  	if (!is_cpuset_online(cs))
>  		goto out_unlock;
> @@ -2422,6 +2427,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>  	free_cpuset(trialcs);
>  out_unlock:
>  	percpu_up_write(&cpuset_rwsem);
> +	put_online_cpus();
>  	kernfs_unbreak_active_protection(of->kn);
>  	css_put(&cs->css);
>  	flush_workqueue(cpuset_migrate_mm_wq);
> @@ -2552,6 +2558,7 @@ static ssize_t sched_partition_write(struct kernfs_open_file *of, char *buf,
>  		return -EINVAL;
>  
>  	css_get(&cs->css);
> +	get_online_cpus();
>  	percpu_down_write(&cpuset_rwsem);
>  	if (!is_cpuset_online(cs))
>  		goto out_unlock;
> @@ -2559,6 +2566,7 @@ static ssize_t sched_partition_write(struct kernfs_open_file *of, char *buf,
>  	retval = update_prstate(cs, val);
>  out_unlock:
>  	percpu_up_write(&cpuset_rwsem);
> +	put_online_cpus();
>  	css_put(&cs->css);
>  	return retval ?: nbytes;
>  }
> @@ -2764,6 +2772,7 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
>  	if (!parent)
>  		return 0;
>  
> +	get_online_cpus();
>  	percpu_down_write(&cpuset_rwsem);
>  
>  	set_bit(CS_ONLINE, &cs->flags);
> @@ -2816,6 +2825,7 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
>  	spin_unlock_irq(&callback_lock);
>  out_unlock:
>  	percpu_up_write(&cpuset_rwsem);
> +	put_online_cpus();
>  	return 0;
>  }
>  
> @@ -2834,6 +2844,7 @@ static void cpuset_css_offline(struct cgroup_subsys_state *css)
>  {
>  	struct cpuset *cs = css_cs(css);
>  
> +	get_online_cpus();
>  	percpu_down_write(&cpuset_rwsem);
>  
>  	if (is_partition_root(cs))
> @@ -2854,6 +2865,7 @@ static void cpuset_css_offline(struct cgroup_subsys_state *css)
>  	clear_bit(CS_ONLINE, &cs->flags);
>  
>  	percpu_up_write(&cpuset_rwsem);
> +	put_online_cpus();
>  }
>  
>  static void cpuset_css_free(struct cgroup_subsys_state *css)
> -- 
> 2.17.2
>