cgroup_freezer: allow moving tasks in and out of a frozen cgroup
cgroup_freezer is one of the few users of cgroup_subsys->can_attach() and uses it to prevent tasks from being migrated into or out of a frozen cgroup. This makes cgroup_freezer cumbersome to use especially when co-mounted with other controllers. ->can_attach() is problematic in general as it can make co-mounting multiple cgroups difficult - migrating tasks may fail for reasons completely irrelevant for other controllers. freezer_can_attach() in particular is more problematic because it messes with cgroup internal locking to ensure that the state verification performed at freezer_can_attach() stays valid until migration is complete. This patch replaces freezer_can_attach() with freezer_attach() so that tasks are always allowed to migrate - they are nudged into the conforming state from freezer_attach(). This means that there can be tasks which are being migrated which don't conform to the current cgroup_freezer state until freezer_attach() is complete. Under the current locking scheme, the only such place is freezer_fork() which is updated to handle such window. While this patch doesn't remove the use of internal cgroup locking from freezer_read/write() paths, it removes the requirement to keep the freezer state constant while migrating and enables such change. Note that this creates a userland visible behavior change - FROZEN cgroup can no longer be used to lock migrations in and out of the cgroup. This behavior change is intended. I don't think the feature is necessary - userland should coordinate accesses to cgroup fs anyway - and even if the feature is needed cgroup_freezer is the completely wrong place to implement it. Signed-off-by: Tejun Heo <tj@kernel.org> LKML-Reference: <1350426526-14254-1-git-send-email-tj@kernel.org> Cc: Matt Helsley <matthltc@linux.vnet.ibm.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Rafael J. Wysocki <rjw@sisk.pl> Cc: Li Zefan <lizefan@huawei.com>
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@ -152,27 +152,38 @@ static void freezer_destroy(struct cgroup *cgroup)
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/*
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/*
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* The call to cgroup_lock() in the freezer.state write method prevents
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* The call to cgroup_lock() in the freezer.state write method prevents
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* a write to that file racing against an attach, and hence the
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* a write to that file racing against an attach, and hence we don't need
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* can_attach() result will remain valid until the attach completes.
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* to worry about racing against migration.
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*/
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*/
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static int freezer_can_attach(struct cgroup *new_cgroup,
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static void freezer_attach(struct cgroup *new_cgrp, struct cgroup_taskset *tset)
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struct cgroup_taskset *tset)
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{
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{
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struct freezer *freezer;
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struct freezer *freezer = cgroup_freezer(new_cgrp);
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struct task_struct *task;
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struct task_struct *task;
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spin_lock_irq(&freezer->lock);
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/*
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/*
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* Anything frozen can't move or be moved to/from.
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* Make the new tasks conform to the current state of @new_cgrp.
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* For simplicity, when migrating any task to a FROZEN cgroup, we
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* revert it to FREEZING and let update_if_frozen() determine the
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* correct state later.
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*
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* Tasks in @tset are on @new_cgrp but may not conform to its
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* current state before executing the following - !frozen tasks may
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* be visible in a FROZEN cgroup and frozen tasks in a THAWED one.
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* This means that, to determine whether to freeze, one should test
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* whether the state equals THAWED.
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*/
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*/
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cgroup_taskset_for_each(task, new_cgroup, tset)
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cgroup_taskset_for_each(task, new_cgrp, tset) {
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if (cgroup_freezing(task))
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if (freezer->state == CGROUP_THAWED) {
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return -EBUSY;
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__thaw_task(task);
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} else {
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freeze_task(task);
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freezer->state = CGROUP_FREEZING;
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}
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}
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freezer = cgroup_freezer(new_cgroup);
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spin_unlock_irq(&freezer->lock);
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if (freezer->state != CGROUP_THAWED)
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return -EBUSY;
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return 0;
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}
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}
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static void freezer_fork(struct task_struct *task)
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static void freezer_fork(struct task_struct *task)
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@ -190,12 +201,12 @@ static void freezer_fork(struct task_struct *task)
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goto out;
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goto out;
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spin_lock_irq(&freezer->lock);
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spin_lock_irq(&freezer->lock);
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BUG_ON(freezer->state == CGROUP_FROZEN);
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/*
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* @task might have been just migrated into a FROZEN cgroup. Test
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/* Locking avoids race with FREEZING -> THAWED transitions. */
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* equality with THAWED. Read the comment in freezer_attach().
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if (freezer->state == CGROUP_FREEZING)
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*/
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if (freezer->state != CGROUP_THAWED)
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freeze_task(task);
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freeze_task(task);
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spin_unlock_irq(&freezer->lock);
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spin_unlock_irq(&freezer->lock);
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out:
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out:
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rcu_read_unlock();
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rcu_read_unlock();
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@ -352,7 +363,7 @@ struct cgroup_subsys freezer_subsys = {
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.create = freezer_create,
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.create = freezer_create,
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.destroy = freezer_destroy,
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.destroy = freezer_destroy,
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.subsys_id = freezer_subsys_id,
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.subsys_id = freezer_subsys_id,
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.can_attach = freezer_can_attach,
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.attach = freezer_attach,
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.fork = freezer_fork,
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.fork = freezer_fork,
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.base_cftypes = files,
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.base_cftypes = files,
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