Re: [PATCH 6/9] cgroup_freezer: make freezer->state mask of flags

[Michal Hocko <mhocko@suse.cz>]

On Sat 03-11-12 01:38:32, Tejun Heo wrote:
> freezer->state was an enum value - one of THAWED, FREEZING and FROZEN.
> As the scheduled full hierarchy support requires more than one
> freezing condition, switch it to mask of flags.  If FREEZING is not
> set, it's thawed.  FREEZING is set if freezing or frozen.  If frozen,
> both FREEZING and FROZEN are set.  Now that tasks can be attached to
> an already frozen cgroup, this also makes freezing condition checks
> more natural.
> 
> This patch doesn't introduce any behavior change.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>

I think it would be nicer to use freezer_state_flags enum rather than
unsigned int for the state. I would even expect gcc to complain about
that but it looks like -fstrict-enums is c++ specific (so long enum
safety).

Anyway
Reviewed-by: Michal Hocko <mhocko@suse.cz>

> ---
>  kernel/cgroup_freezer.c | 60 ++++++++++++++++++++++---------------------------
>  1 file changed, 27 insertions(+), 33 deletions(-)
> 
> diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
> index 2690830..e76aa9f 100644
> --- a/kernel/cgroup_freezer.c
> +++ b/kernel/cgroup_freezer.c
> @@ -22,15 +22,14 @@
>  #include <linux/freezer.h>
>  #include <linux/seq_file.h>
>  
> -enum freezer_state {
> -	CGROUP_THAWED = 0,
> -	CGROUP_FREEZING,
> -	CGROUP_FROZEN,
> +enum freezer_state_flags {
> +	CGROUP_FREEZING		= (1 << 1), /* this freezer is freezing */
> +	CGROUP_FROZEN		= (1 << 3), /* this and its descendants frozen */
>  };
>  
>  struct freezer {
>  	struct cgroup_subsys_state	css;
> -	enum freezer_state		state;
> +	unsigned int			state;
>  	spinlock_t			lock;
>  };
>  
> @@ -48,12 +47,10 @@ static inline struct freezer *task_freezer(struct task_struct *task)
>  
>  bool cgroup_freezing(struct task_struct *task)
>  {
> -	enum freezer_state state;
>  	bool ret;
>  
>  	rcu_read_lock();
> -	state = task_freezer(task)->state;
> -	ret = state == CGROUP_FREEZING || state == CGROUP_FROZEN;
> +	ret = task_freezer(task)->state & CGROUP_FREEZING;
>  	rcu_read_unlock();
>  
>  	return ret;
> @@ -63,10 +60,13 @@ bool cgroup_freezing(struct task_struct *task)
>   * cgroups_write_string() limits the size of freezer state strings to
>   * CGROUP_LOCAL_BUFFER_SIZE
>   */
> -static const char *freezer_state_strs[] = {
> -	"THAWED",
> -	"FREEZING",
> -	"FROZEN",
> +static const char *freezer_state_strs(unsigned int state)
> +{
> +	if (state & CGROUP_FROZEN)
> +		return "FROZEN";
> +	if (state & CGROUP_FREEZING)
> +		return "FREEZING";
> +	return "THAWED";
>  };
>  
>  /*
> @@ -91,7 +91,6 @@ static struct cgroup_subsys_state *freezer_create(struct cgroup *cgroup)
>  		return ERR_PTR(-ENOMEM);
>  
>  	spin_lock_init(&freezer->lock);
> -	freezer->state = CGROUP_THAWED;
>  	return &freezer->css;
>  }
>  
> @@ -99,7 +98,7 @@ static void freezer_destroy(struct cgroup *cgroup)
>  {
>  	struct freezer *freezer = cgroup_freezer(cgroup);
>  
> -	if (freezer->state != CGROUP_THAWED)
> +	if (freezer->state & CGROUP_FREEZING)
>  		atomic_dec(&system_freezing_cnt);
>  	kfree(freezer);
>  }
> @@ -129,15 +128,13 @@ static void freezer_attach(struct cgroup *new_cgrp, struct cgroup_taskset *tset)
>  	 * Tasks in @tset are on @new_cgrp but may not conform to its
>  	 * current state before executing the following - !frozen tasks may
>  	 * be visible in a FROZEN cgroup and frozen tasks in a THAWED one.
> -	 * This means that, to determine whether to freeze, one should test
> -	 * whether the state equals THAWED.
>  	 */
>  	cgroup_taskset_for_each(task, new_cgrp, tset) {
> -		if (freezer->state == CGROUP_THAWED) {
> +		if (!(freezer->state & CGROUP_FREEZING)) {
>  			__thaw_task(task);
>  		} else {
>  			freeze_task(task);
> -			freezer->state = CGROUP_FREEZING;
> +			freezer->state &= ~CGROUP_FROZEN;
>  		}
>  	}
>  
> @@ -159,11 +156,7 @@ static void freezer_fork(struct task_struct *task)
>  		goto out;
>  
>  	spin_lock_irq(&freezer->lock);
> -	/*
> -	 * @task might have been just migrated into a FROZEN cgroup.  Test
> -	 * equality with THAWED.  Read the comment in freezer_attach().
> -	 */
> -	if (freezer->state != CGROUP_THAWED)
> +	if (freezer->state & CGROUP_FREEZING)
>  		freeze_task(task);
>  	spin_unlock_irq(&freezer->lock);
>  out:
> @@ -184,7 +177,8 @@ static void update_if_frozen(struct freezer *freezer)
>  	struct cgroup_iter it;
>  	struct task_struct *task;
>  
> -	if (freezer->state != CGROUP_FREEZING)
> +	if (!(freezer->state & CGROUP_FREEZING) ||
> +	    (freezer->state & CGROUP_FROZEN))
>  		return;
>  
>  	cgroup_iter_start(cgroup, &it);
> @@ -202,7 +196,7 @@ static void update_if_frozen(struct freezer *freezer)
>  		}
>  	}
>  
> -	freezer->state = CGROUP_FROZEN;
> +	freezer->state |= CGROUP_FROZEN;
>  notyet:
>  	cgroup_iter_end(cgroup, &it);
>  }
> @@ -211,14 +205,14 @@ static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
>  			struct seq_file *m)
>  {
>  	struct freezer *freezer = cgroup_freezer(cgroup);
> -	enum freezer_state state;
> +	unsigned int state;
>  
>  	spin_lock_irq(&freezer->lock);
>  	update_if_frozen(freezer);
>  	state = freezer->state;
>  	spin_unlock_irq(&freezer->lock);
>  
> -	seq_puts(m, freezer_state_strs[state]);
> +	seq_puts(m, freezer_state_strs(state));
>  	seq_putc(m, '\n');
>  	return 0;
>  }
> @@ -258,14 +252,14 @@ static void freezer_apply_state(struct freezer *freezer, bool freeze)
>  	lockdep_assert_held(&freezer->lock);
>  
>  	if (freeze) {
> -		if (freezer->state == CGROUP_THAWED)
> +		if (!(freezer->state & CGROUP_FREEZING))
>  			atomic_inc(&system_freezing_cnt);
> -		freezer->state = CGROUP_FREEZING;
> +		freezer->state |= CGROUP_FREEZING;
>  		freeze_cgroup(freezer);
>  	} else {
> -		if (freezer->state != CGROUP_THAWED)
> +		if (freezer->state & CGROUP_FREEZING)
>  			atomic_dec(&system_freezing_cnt);
> -		freezer->state = CGROUP_THAWED;
> +		freezer->state &= ~(CGROUP_FREEZING | CGROUP_FROZEN);
>  		unfreeze_cgroup(freezer);
>  	}
>  }
> @@ -290,9 +284,9 @@ static int freezer_write(struct cgroup *cgroup, struct cftype *cft,
>  {
>  	bool freeze;
>  
> -	if (strcmp(buffer, freezer_state_strs[CGROUP_THAWED]) == 0)
> +	if (strcmp(buffer, freezer_state_strs(0)) == 0)
>  		freeze = false;
> -	else if (strcmp(buffer, freezer_state_strs[CGROUP_FROZEN]) == 0)
> +	else if (strcmp(buffer, freezer_state_strs(CGROUP_FROZEN)) == 0)
>  		freeze = true;
>  	else
>  		return -EINVAL;
> -- 
> 1.7.11.7
> 

-- 
Michal Hocko
SUSE Labs