linux-kernel - Re: [PATCH 1/8] cgroup: kill cgroup_subsys->__DEPRECATED_clear_css

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Date:	Wed, 31 Oct 2012 17:21:29 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<lizefan@...wei.com>, <hannes@...xchg.org>, <mhocko@...e.cz>,
	<bsingharora@...il.com>, <kamezawa.hiroyu@...fujitsu.com>,
	<cgroups@...r.kernel.org>, <containers@...ts.linux-foundation.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/8] cgroup: kill cgroup_subsys->__DEPRECATED_clear_css_refs


>  	}
>  	prepare_to_wait(&cgroup_rmdir_waitq, &wait, TASK_INTERRUPTIBLE);
> -	if (!cgroup_clear_css_refs(cgrp)) {
> -		mutex_unlock(&cgroup_mutex);
> -		/*
> -		 * Because someone may call cgroup_wakeup_rmdir_waiter() before
> -		 * prepare_to_wait(), we need to check this flag.
> -		 */
> -		if (test_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags))
> -			schedule();
> -		finish_wait(&cgroup_rmdir_waitq, &wait);
> -		clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
> -		if (signal_pending(current))
> -			return -EINTR;
> -		goto again;
> +
> +	local_irq_disable();
> +
> +	/* block new css_tryget() by deactivating refcnt */
> +	for_each_subsys(cgrp->root, ss) {
> +		struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
> +
> +		WARN_ON(atomic_read(&css->refcnt) < 0);
> +		atomic_add(CSS_DEACT_BIAS, &css->refcnt);
>  	}
> -	/* NO css_tryget() can success after here. */
> +
> +	/*
> +	 * Set REMOVED.  All in-progress css_tryget() will be released.
> +	 * Put all the base refs.  Each css holds an extra reference to the
> +	 * cgroup's dentry and cgroup removal proceeds regardless of css
> +	 * refs.  On the last put of each css, whenever that may be, the
> +	 * extra dentry ref is put so that dentry destruction happens only
> +	 * after all css's are released.
> +	 */
> +	for_each_subsys(cgrp->root, ss) {
> +		struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
> +
> +		set_bit(CSS_REMOVED, &css->flags);
> +		css_put(css);
> +	}
> +
> +	local_irq_enable();
> +
>  	finish_wait(&cgroup_rmdir_waitq, &wait);

It seems to me that the main reason for doing this in two passes was to
derive a global "failed" state on the first pass. Now that we can't
fail, why can't you just loop through the subsystems just once ?

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