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cgroup_freezer: update_freezer_state() does incorrect state transitions

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There are 4 state transitions possible for a freezer.  Only FREEZING ->
FROZEN transaction is done lazily.  This patch allows update_freezer_state
only to perform this transaction and renames the function to
update_if_frozen.

Moreover is_task_frozen_enough function is removed and its every occurence
is replaced with frozen().  Therefore for a group to become FROZEN every
task must be frozen.

The previous version could trigger a following bug: When cgroup is in the
process of freezing (but none of its tasks are frozen yet),
update_freezer_state() (called from freezer_read or freezer_write) would
incorrectly report that a group is 'THAWED' (because nfrozen = 0),
allowing the transaction FREEZING -> THAWED without writing anything to
'freezer.state'.  This is incorrect according to the documentation.  This
could result in a 'THAWED' cgroup with frozen tasks inside.

A code to reproduce this bug is available here:
http://pentium.hopto.org/~thinred/repos/linux-misc/freezer_bug2.c

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Tomasz Buchert <tomasz.buchert@inria.fr>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Tomasz Buchert 2010-10-27 15:33:34 -07:00 committed by Linus Torvalds
parent 0bdba580ab
commit 2d3cbf8bc8
1 changed files with 15 additions and 23 deletions
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38
kernel/cgroup_freezer.c
View file

@ -153,13 +153,6 @@ static void freezer_destroy(struct cgroup_subsys *ss,
kfree(cgroup_freezer(cgroup));
}
/* Task is frozen or will freeze immediately when next it gets woken */
static bool is_task_frozen_enough(struct task_struct *task)
{
return frozen(task) ||
(task_is_stopped_or_traced(task) && freezing(task));
}
/*
* The call to cgroup_lock() in the freezer.state write method prevents
* a write to that file racing against an attach, and hence the
@ -236,31 +229,30 @@ static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
/*
* caller must hold freezer->lock
*/
static void update_freezer_state(struct cgroup *cgroup,
static void update_if_frozen(struct cgroup *cgroup,
struct freezer *freezer)
{
struct cgroup_iter it;
struct task_struct *task;
unsigned int nfrozen = 0, ntotal = 0;
enum freezer_state old_state = freezer->state;
cgroup_iter_start(cgroup, &it);
while ((task = cgroup_iter_next(cgroup, &it))) {
ntotal++;
if (is_task_frozen_enough(task))
if (frozen(task))
nfrozen++;
}
/*
* Transition to FROZEN when no new tasks can be added ensures
* that we never exist in the FROZEN state while there are unfrozen
* tasks.
*/
if (nfrozen == ntotal)
freezer->state = CGROUP_FROZEN;
else if (nfrozen > 0)
freezer->state = CGROUP_FREEZING;
else
freezer->state = CGROUP_THAWED;
if (old_state == CGROUP_THAWED) {
BUG_ON(nfrozen > 0);
} else if (old_state == CGROUP_FREEZING) {
if (nfrozen == ntotal)
freezer->state = CGROUP_FROZEN;
} else { /* old_state == CGROUP_FROZEN */
BUG_ON(nfrozen != ntotal);
}
cgroup_iter_end(cgroup, &it);
}
@ -279,7 +271,7 @@ static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
if (state == CGROUP_FREEZING) {
/* We change from FREEZING to FROZEN lazily if the cgroup was
* only partially frozen when we exitted write. */
update_freezer_state(cgroup, freezer);
update_if_frozen(cgroup, freezer);
state = freezer->state;
}
spin_unlock_irq(&freezer->lock);
@ -301,7 +293,7 @@ static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
while ((task = cgroup_iter_next(cgroup, &it))) {
if (!freeze_task(task, true))
continue;
if (is_task_frozen_enough(task))
if (frozen(task))
continue;
if (!freezing(task) && !freezer_should_skip(task))
num_cant_freeze_now++;
@ -335,7 +327,7 @@ static int freezer_change_state(struct cgroup *cgroup,
spin_lock_irq(&freezer->lock);
update_freezer_state(cgroup, freezer);
update_if_frozen(cgroup, freezer);
if (goal_state == freezer->state)
goto out;