From e5ced8ebb10c20a3b349bd798b69ccabd3b25d21 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Wed, 7 May 2014 21:31:17 -0400 Subject: 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: [] vfs_write+0x1a3/0x1c0 #1: (&of->mutex){+.+.+.}, at: [] kernfs_fop_write+0xbb/0x170 #2: (s_active#70){.+.+.+}, at: [] kernfs_fop_write+0xc3/0x170 #3: (freezer_mutex){+.+...}, at: [] freezer_write+0x61/0x1e0 #4: (rcu_read_lock){......}, at: [] freezer_write+0x53/0x1e0 Preemption disabled at:[] 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: [] dump_stack+0x4e/0x7a [] __might_sleep+0x162/0x260 [] down_read+0x24/0x60 [] css_task_iter_start+0x27/0x70 [] freezer_apply_state+0x5d/0x130 [] freezer_write+0xf6/0x1e0 [] cgroup_file_write+0xd8/0x230 [] kernfs_fop_write+0xe7/0x170 [] vfs_write+0xb6/0x1c0 [] SyS_write+0x4d/0xc0 [] 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 Acked-by: Li Zefan --- kernel/cgroup_freezer.c | 112 ++++++++++++++++++++---------------------------- 1 file changed, 46 insertions(+), 66 deletions(-) diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c index 2bc4a225644..12ead0b766e 100644 --- a/kernel/cgroup_freezer.c +++ b/kernel/cgroup_freezer.c @@ -21,6 +21,7 @@ #include #include #include +#include /* * A cgroup is freezing if any FREEZING flags are set. FREEZING_SELF is @@ -42,9 +43,10 @@ enum freezer_state_flags { 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 @@ freezer_css_alloc(struct cgroup_subsys_state *parent_css) if (!freezer) return ERR_PTR(-ENOMEM); - spin_lock_init(&freezer->lock); return &freezer->css; } @@ -110,14 +111,7 @@ static int freezer_css_online(struct cgroup_subsys_state *css) 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 @@ static int freezer_css_online(struct cgroup_subsys_state *css) 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 @@ static void freezer_css_offline(struct cgroup_subsys_state *css) { 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 @@ static void freezer_attach(struct cgroup_subsys_state *new_css, 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 @@ static void freezer_attach(struct cgroup_subsys_state *new_css, } } - 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 @@ static void freezer_fork(struct task_struct *task) { 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 @@ static void freezer_fork(struct task_struct *task) * 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 @@ static void update_if_frozen(struct cgroup_subsys_state *css) 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 @@ static void update_if_frozen(struct cgroup_subsys_state *css) 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 @@ static void freezer_apply_state(struct freezer *freezer, bool freeze, 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 @@ static void freezer_change_state(struct freezer *freezer, bool freeze) * 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, -- cgit v1.2.3-70-g09d2