Re: [PATCH pm-freezer 1/4] cgroup_freezer: fix freezer->state setting bug in freezer_change_state()

[Matt Helsley <matthltc@us.ibm.com>]

On Wed, Aug 31, 2011 at 12:21:07PM +0200, Tejun Heo wrote:
> d02f52811d0e "cgroup_freezer: prepare for removal of TIF_FREEZE" moved
> setting of freezer->state into freezer_change_state(); unfortunately,
> while doing so, when it's beginning to freeze tasks, it sets the state
> to CGROUP_FROZEN instead of CGROUP_FREEZING ending up skipping the
> whole freezing state.  Fix it.
> 
> -v2: Oleg pointed out that re-freezing FROZEN cgroup could increment
>      system_freezing_cnt.  Fixed.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reported-by: Oleg Nesterov <oleg@redhat.com>
> Cc: Paul Menage <paul@paulmenage.org>
> Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
> ---
> I'm in the process of moving and can only use a quite old laptop.  I
> tested compile but couldn't really do much else, so please proceed
> with caution.  Oleg, can you please ack the patches if you agree with
> the updated versions?
> 
> Thanks.
> 
>  kernel/cgroup_freezer.c |   20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> Index: work/kernel/cgroup_freezer.c
> ===================================================================
> --- work.orig/kernel/cgroup_freezer.c
> +++ work/kernel/cgroup_freezer.c
> @@ -308,24 +308,26 @@ static int freezer_change_state(struct c
>  	spin_lock_irq(&freezer->lock);
> 
>  	update_if_frozen(cgroup, freezer);
> -	if (goal_state == freezer->state)
> -		goto out;
> -
> -	freezer->state = goal_state;
> 
>  	switch (goal_state) {
>  	case CGROUP_THAWED:
> -		atomic_dec(&system_freezing_cnt);
> -		unfreeze_cgroup(cgroup, freezer);
> +		if (freezer->state != CGROUP_THAWED) {
> +			freezer->state = CGROUP_THAWED;
> +			atomic_dec(&system_freezing_cnt);
> +			unfreeze_cgroup(cgroup, freezer);
> +		}
>  		break;
>  	case CGROUP_FROZEN:
> -		atomic_inc(&system_freezing_cnt);
> -		retval = try_to_freeze_cgroup(cgroup, freezer);
> +		if (freezer->state == CGROUP_THAWED) {
> +			freezer->state = CGROUP_FREEZING;
> +			atomic_inc(&system_freezing_cnt);
> +			retval = try_to_freeze_cgroup(cgroup, freezer);

This still doesn't look quite right. If the cgroup is FREEZING it should
also call try_to_freeze_cgroup(). I think this is what's needed:

		if (freezer->state == CGROUP_THAWED)
			atomic_inc(&system_freezing_cnt);
		freezer->state = CGROUP_FREEZING;
		retval = try_to_freeze_cgroup(cgroup, freezer);

> +		}
>  		break;
>  	default:
>  		BUG();
>  	}

Cheers,
	-Matt Helsley