Commit e9316080 authored by Tejun Heo's avatar Tejun Heo
Browse files

cgroup: kill CSS_REMOVED 



CSS_REMOVED is one of the several contortions which were necessary to
support css reference draining on cgroup removal.  All css->refcnts
which need draining should be deactivated and verified to equal zero
atomically w.r.t. css_tryget().  If any one isn't zero, all refcnts
needed to be re-activated and css_tryget() shouldn't fail in the
process.

This was achieved by letting css_tryget() busy-loop until either the
refcnt is reactivated (failed removal attempt) or CSS_REMOVED is set
(committing to removal).

Now that css refcnt draining is no longer used, there's no need for
atomic rollback mechanism.  css_tryget() simply can look at the
reference count and fail if it's deactivated - it's never getting
re-activated.

This patch removes CSS_REMOVED and updates __css_tryget() to fail if
the refcnt is deactivated.  As deactivation and removal are a single
step now, they no longer need to be protected against css_tryget()
happening from irq context.  Remove local_irq_disable/enable() from
cgroup_rmdir().

Note that this removes css_is_removed() whose only user is VM_BUG_ON()
in memcontrol.c.  We can replace it with a check on the refcnt but
given that the only use case is a debug assert, I think it's better to
simply unexport it.

v2: Comment updated and explanation on local_irq_disable/enable()
    added per Michal Hocko.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Reviewed-by: default avatarMichal Hocko <mhocko@suse.cz>
Reviewed-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: default avatarLi Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
parent ed957793

include/linux/cgroup.h

+0 −6
Original line number Diff line number Diff line
@@ -85,7 +85,6 @@ struct cgroup_subsys_state { 
/* bits in struct cgroup_subsys_state flags field */

enum {

	CSS_ROOT, /* This CSS is the root of the subsystem */

	CSS_REMOVED, /* This CSS is dead */

};



/* Caller must verify that the css is not for root cgroup */
@@ -108,11 +107,6 @@ static inline void css_get(struct cgroup_subsys_state *css) 
		__css_get(css, 1);

}



static inline bool css_is_removed(struct cgroup_subsys_state *css)

{

	return test_bit(CSS_REMOVED, &css->flags);

}



/*

 * Call css_tryget() to take a reference on a css if your existing

 * (known-valid) reference isn't already ref-counted. Returns false if

kernel/cgroup.c

+12 −19
Original line number Diff line number Diff line
@@ -170,8 +170,8 @@ struct css_id { 
	 * The css to which this ID points. This pointer is set to valid value

	 * after cgroup is populated. If cgroup is removed, this will be NULL.

	 * This pointer is expected to be RCU-safe because destroy()

	 * is called after synchronize_rcu(). But for safe use, css_is_removed()

	 * css_tryget() should be used for avoiding race.

	 * is called after synchronize_rcu(). But for safe use, css_tryget()

	 * should be used for avoiding race.

	 */

	struct cgroup_subsys_state __rcu *css;

	/*
@@ -4112,8 +4112,6 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry) 
	}

	prepare_to_wait(&cgroup_rmdir_waitq, &wait, TASK_INTERRUPTIBLE);



	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];
@@ -4123,21 +4121,14 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry) 
	}



	/*

	 * 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();

	for_each_subsys(cgrp->root, ss)

		css_put(cgrp->subsys[ss->subsys_id]);



	finish_wait(&cgroup_rmdir_waitq, &wait);

	clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
@@ -4861,15 +4852,17 @@ static void check_for_release(struct cgroup *cgrp) 
/* Caller must verify that the css is not for root cgroup */

bool __css_tryget(struct cgroup_subsys_state *css)

{

	do {

		int v = css_refcnt(css);

	while (true) {

		int t, v;



		if (atomic_cmpxchg(&css->refcnt, v, v + 1) == v)

		v = css_refcnt(css);

		t = atomic_cmpxchg(&css->refcnt, v, v + 1);

		if (likely(t == v))

			return true;

		cpu_relax();

	} while (!test_bit(CSS_REMOVED, &css->flags));



		else if (t < 0)

			return false;

		cpu_relax();

	}

}

EXPORT_SYMBOL_GPL(__css_tryget);

mm/memcontrol.c

+3 −4
Original line number Diff line number Diff line
@@ -2343,7 +2343,6 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm, 
again:

	if (*ptr) { /* css should be a valid one */

		memcg = *ptr;

		VM_BUG_ON(css_is_removed(&memcg->css));

		if (mem_cgroup_is_root(memcg))

			goto done;

		if (nr_pages == 1 && consume_stock(memcg))
@@ -2483,9 +2482,9 @@ static void __mem_cgroup_cancel_local_charge(struct mem_cgroup *memcg, 


/*

 * A helper function to get mem_cgroup from ID. must be called under

 * rcu_read_lock(). The caller must check css_is_removed() or some if

 * it's concern. (dropping refcnt from swap can be called against removed

 * memcg.)

 * rcu_read_lock().  The caller is responsible for calling css_tryget if

 * the mem_cgroup is used for charging. (dropping refcnt from swap can be

 * called against removed memcg.)

 */

static struct mem_cgroup *mem_cgroup_lookup(unsigned short id)

{

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