cgroup_freezer: replace freezer->lock with freezer_mutex
After 96d365e0b86e ("cgroup: make css_set_lock a rwsem and rename it
to css_set_rwsem"), css task iterators requires sleepable context as
it may block on css_set_rwsem. I missed that cgroup_freezer was
iterating tasks under IRQ-safe spinlock freezer->lock. This leads to
errors like the following on freezer state reads and transitions.
BUG: sleeping function called from invalid context at /work
/os/work/kernel/locking/rwsem.c:20
in_atomic(): 0, irqs_disabled(): 0, pid: 462, name: bash
5 locks held by bash/462:
#0: (sb_writers#7){.+.+.+}, at: [<ffffffff811f0843>] vfs_write+0x1a3/0x1c0
#1: (&of->mutex){+.+.+.}, at: [<ffffffff8126d78b>] kernfs_fop_write+0xbb/0x170
#2: (s_active#70){.+.+.+}, at: [<ffffffff8126d793>] kernfs_fop_write+0xc3/0x170
#3: (freezer_mutex){+.+...}, at: [<ffffffff81135981>] freezer_write+0x61/0x1e0
#4: (rcu_read_lock){......}, at: [<ffffffff81135973>] freezer_write+0x53/0x1e0
Preemption disabled at:[<ffffffff81104404>] console_unlock+0x1e4/0x460
CPU: 3 PID: 462 Comm: bash Not tainted 3.15.0-rc1-work+ #10
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
ffff88000916a6d0 ffff88000e0a3da0 ffffffff81cf8c96 0000000000000000
ffff88000e0a3dc8 ffffffff810cf4f2 ffffffff82388040 ffff880013aaf740
0000000000000002 ffff88000e0a3de8 ffffffff81d05974 0000000000000246
Call Trace:
[<ffffffff81cf8c96>] dump_stack+0x4e/0x7a
[<ffffffff810cf4f2>] __might_sleep+0x162/0x260
[<ffffffff81d05974>] down_read+0x24/0x60
[<ffffffff81133e87>] css_task_iter_start+0x27/0x70
[<ffffffff8113584d>] freezer_apply_state+0x5d/0x130
[<ffffffff81135a16>] freezer_write+0xf6/0x1e0
[<ffffffff8112eb88>] cgroup_file_write+0xd8/0x230
[<ffffffff8126d7b7>] kernfs_fop_write+0xe7/0x170
[<ffffffff811f0756>] vfs_write+0xb6/0x1c0
[<ffffffff811f121d>] SyS_write+0x4d/0xc0
[<ffffffff81d08292>] system_call_fastpath+0x16/0x1b
freezer->lock used to be used in hot paths but that time is long gone
and there's no reason for the lock to be IRQ-safe spinlock or even
per-cgroup. In fact, given the fact that a cgroup may contain large
number of tasks, it's not a good idea to iterate over them while
holding IRQ-safe spinlock.
Let's simplify locking by replacing per-cgroup freezer->lock with
global freezer_mutex. This also makes the comments explaining the
intricacies of policy inheritance and the locking around it as the
states are protected by a common mutex.
The conversion is mostly straight-forward. The followings are worth
mentioning.
* freezer_css_online() no longer needs double locking.
* freezer_attach() now performs propagation simply while holding
freezer_mutex. update_if_frozen() race no longer exists and the
comment is removed.
* freezer_fork() now tests whether the task is in root cgroup using
the new task_css_is_root() without doing rcu_read_lock/unlock(). If
not, it grabs freezer_mutex and performs the operation.
* freezer_read() and freezer_change_state() grab freezer_mutex across
the whole operation and pin the css while iterating so that each
descendant processing happens in sleepable context.
Fixes: 96d365e0b86e ("cgroup: make css_set_lock a rwsem and rename it to css_set_rwsem")
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 2bc4a22..12ead0b 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -21,6 +21,7 @@
#include <linux/uaccess.h>
#include <linux/freezer.h>
#include <linux/seq_file.h>
+#include <linux/mutex.h>
/*
* A cgroup is freezing if any FREEZING flags are set. FREEZING_SELF is
@@ -42,9 +43,10 @@
struct freezer {
struct cgroup_subsys_state css;
unsigned int state;
- spinlock_t lock;
};
+static DEFINE_MUTEX(freezer_mutex);
+
static inline struct freezer *css_freezer(struct cgroup_subsys_state *css)
{
return css ? container_of(css, struct freezer, css) : NULL;
@@ -93,7 +95,6 @@
if (!freezer)
return ERR_PTR(-ENOMEM);
- spin_lock_init(&freezer->lock);
return &freezer->css;
}
@@ -110,14 +111,7 @@
struct freezer *freezer = css_freezer(css);
struct freezer *parent = parent_freezer(freezer);
- /*
- * The following double locking and freezing state inheritance
- * guarantee that @cgroup can never escape ancestors' freezing
- * states. See css_for_each_descendant_pre() for details.
- */
- if (parent)
- spin_lock_irq(&parent->lock);
- spin_lock_nested(&freezer->lock, SINGLE_DEPTH_NESTING);
+ mutex_lock(&freezer_mutex);
freezer->state |= CGROUP_FREEZER_ONLINE;
@@ -126,10 +120,7 @@
atomic_inc(&system_freezing_cnt);
}
- spin_unlock(&freezer->lock);
- if (parent)
- spin_unlock_irq(&parent->lock);
-
+ mutex_unlock(&freezer_mutex);
return 0;
}
@@ -144,14 +135,14 @@
{
struct freezer *freezer = css_freezer(css);
- spin_lock_irq(&freezer->lock);
+ mutex_lock(&freezer_mutex);
if (freezer->state & CGROUP_FREEZING)
atomic_dec(&system_freezing_cnt);
freezer->state = 0;
- spin_unlock_irq(&freezer->lock);
+ mutex_unlock(&freezer_mutex);
}
static void freezer_css_free(struct cgroup_subsys_state *css)
@@ -175,7 +166,7 @@
struct task_struct *task;
bool clear_frozen = false;
- spin_lock_irq(&freezer->lock);
+ mutex_lock(&freezer_mutex);
/*
* Make the new tasks conform to the current state of @new_css.
@@ -197,21 +188,13 @@
}
}
- spin_unlock_irq(&freezer->lock);
-
- /*
- * Propagate FROZEN clearing upwards. We may race with
- * update_if_frozen(), but as long as both work bottom-up, either
- * update_if_frozen() sees child's FROZEN cleared or we clear the
- * parent's FROZEN later. No parent w/ !FROZEN children can be
- * left FROZEN.
- */
+ /* propagate FROZEN clearing upwards */
while (clear_frozen && (freezer = parent_freezer(freezer))) {
- spin_lock_irq(&freezer->lock);
freezer->state &= ~CGROUP_FROZEN;
clear_frozen = freezer->state & CGROUP_FREEZING;
- spin_unlock_irq(&freezer->lock);
}
+
+ mutex_unlock(&freezer_mutex);
}
/**
@@ -228,9 +211,6 @@
{
struct freezer *freezer;
- rcu_read_lock();
- freezer = task_freezer(task);
-
/*
* The root cgroup is non-freezable, so we can skip locking the
* freezer. This is safe regardless of race with task migration.
@@ -238,24 +218,18 @@
* to do. If we lost and root is the new cgroup, noop is still the
* right thing to do.
*/
- if (!parent_freezer(freezer))
- goto out;
+ if (task_css_is_root(task, freezer_cgrp_id))
+ return;
- /*
- * Grab @freezer->lock and freeze @task after verifying @task still
- * belongs to @freezer and it's freezing. The former is for the
- * case where we have raced against task migration and lost and
- * @task is already in a different cgroup which may not be frozen.
- * This isn't strictly necessary as freeze_task() is allowed to be
- * called spuriously but let's do it anyway for, if nothing else,
- * documentation.
- */
- spin_lock_irq(&freezer->lock);
- if (freezer == task_freezer(task) && (freezer->state & CGROUP_FREEZING))
+ mutex_lock(&freezer_mutex);
+ rcu_read_lock();
+
+ freezer = task_freezer(task);
+ if (freezer->state & CGROUP_FREEZING)
freeze_task(task);
- spin_unlock_irq(&freezer->lock);
-out:
+
rcu_read_unlock();
+ mutex_unlock(&freezer_mutex);
}
/**
@@ -281,22 +255,22 @@
struct css_task_iter it;
struct task_struct *task;
- WARN_ON_ONCE(!rcu_read_lock_held());
-
- spin_lock_irq(&freezer->lock);
+ lockdep_assert_held(&freezer_mutex);
if (!(freezer->state & CGROUP_FREEZING) ||
(freezer->state & CGROUP_FROZEN))
- goto out_unlock;
+ return;
/* are all (live) children frozen? */
+ rcu_read_lock();
css_for_each_child(pos, css) {
struct freezer *child = css_freezer(pos);
if ((child->state & CGROUP_FREEZER_ONLINE) &&
!(child->state & CGROUP_FROZEN))
- goto out_unlock;
+ return;
}
+ rcu_read_unlock();
/* are all tasks frozen? */
css_task_iter_start(css, &it);
@@ -317,21 +291,29 @@
freezer->state |= CGROUP_FROZEN;
out_iter_end:
css_task_iter_end(&it);
-out_unlock:
- spin_unlock_irq(&freezer->lock);
}
static int freezer_read(struct seq_file *m, void *v)
{
struct cgroup_subsys_state *css = seq_css(m), *pos;
+ mutex_lock(&freezer_mutex);
rcu_read_lock();
/* update states bottom-up */
- css_for_each_descendant_post(pos, css)
+ css_for_each_descendant_post(pos, css) {
+ if (!css_tryget(pos))
+ continue;
+ rcu_read_unlock();
+
update_if_frozen(pos);
+ rcu_read_lock();
+ css_put(pos);
+ }
+
rcu_read_unlock();
+ mutex_unlock(&freezer_mutex);
seq_puts(m, freezer_state_strs(css_freezer(css)->state));
seq_putc(m, '\n');
@@ -373,7 +355,7 @@
unsigned int state)
{
/* also synchronizes against task migration, see freezer_attach() */
- lockdep_assert_held(&freezer->lock);
+ lockdep_assert_held(&freezer_mutex);
if (!(freezer->state & CGROUP_FREEZER_ONLINE))
return;
@@ -414,31 +396,29 @@
* descendant will try to inherit its parent's FREEZING state as
* CGROUP_FREEZING_PARENT.
*/
+ mutex_lock(&freezer_mutex);
rcu_read_lock();
css_for_each_descendant_pre(pos, &freezer->css) {
struct freezer *pos_f = css_freezer(pos);
struct freezer *parent = parent_freezer(pos_f);
- spin_lock_irq(&pos_f->lock);
+ if (!css_tryget(pos))
+ continue;
+ rcu_read_unlock();
- if (pos_f == freezer) {
+ if (pos_f == freezer)
freezer_apply_state(pos_f, freeze,
CGROUP_FREEZING_SELF);
- } else {
- /*
- * Our update to @parent->state is already visible
- * which is all we need. No need to lock @parent.
- * For more info on synchronization, see
- * freezer_post_create().
- */
+ else
freezer_apply_state(pos_f,
parent->state & CGROUP_FREEZING,
CGROUP_FREEZING_PARENT);
- }
- spin_unlock_irq(&pos_f->lock);
+ rcu_read_lock();
+ css_put(pos);
}
rcu_read_unlock();
+ mutex_unlock(&freezer_mutex);
}
static int freezer_write(struct cgroup_subsys_state *css, struct cftype *cft,