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cgroup: Fix sleeping from invalid context warning on PREEMPT_RT

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cgroup_task_dead() is called from finish_task_switch() which runs with
preemption disabled and doesn't allow scheduling even on PREEMPT_RT. The
function needs to acquire css_set_lock which is a regular spinlock that can
sleep on RT kernels, leading to "sleeping function called from invalid
context" warnings.

css_set_lock is too large in scope to convert to a raw_spinlock. However,
the unlinking operations don't need to run synchronously - they just need
to complete after the task is done running.

On PREEMPT_RT, defer the work through irq_work. While the work doesn't need
to happen immediately, it can't be delayed indefinitely either as the dead
task pins the cgroup and task_struct can be pinned indefinitely. Use the
lazy version of irq_work to allow batching and lower impact while ensuring
timely completion.

v2: Use IRQ_WORK_INIT_LAZY instead of immediate irq_work and add explanation
    for why the work can't be delayed indefinitely (Sebastian Andrzej Siewior).

Fixes: d245698d72 ("cgroup: Defer task cgroup unlink until after the task is done switching out")
Reported-by: Calvin Owens <calvin@wbinvd.org>
Link: https://lore.kernel.org/r/20251104181114.489391-1-calvin@wbinvd.org
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tejun Heo
2025-11-06 08:12:36 -10:00
parent be04e96ba9
commit 9311e6c29b
2 changed files with 58 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

5
include/linux/sched.h
View File

@@ -1324,7 +1324,10 @@ struct task_struct {
struct css_set __rcu *cgroups;
/* cg_list protected by css_set_lock and tsk->alloc_lock: */
struct list_head cg_list;
#endif
#ifdef CONFIG_PREEMPT_RT
struct llist_node cg_dead_lnode;
#endif /* CONFIG_PREEMPT_RT */
#endif /* CONFIG_CGROUPS */
#ifdef CONFIG_X86_CPU_RESCTRL
u32 closid;
u32 rmid;

55
kernel/cgroup/cgroup.c
View File

@@ -290,6 +290,7 @@ static void kill_css(struct cgroup_subsys_state *css);
static int cgroup_addrm_files(struct cgroup_subsys_state *css,
struct cgroup *cgrp, struct cftype cfts[],
bool is_add);
static void cgroup_rt_init(void);
#ifdef CONFIG_DEBUG_CGROUP_REF
#define CGROUP_REF_FN_ATTRS noinline
@@ -6360,6 +6361,7 @@ int __init cgroup_init(void)
BUG_ON(ss_rstat_init(NULL));
get_user_ns(init_cgroup_ns.user_ns);
cgroup_rt_init();
cgroup_lock();
@@ -6990,7 +6992,7 @@ void cgroup_task_exit(struct task_struct *tsk)
} while_each_subsys_mask();
}
void cgroup_task_dead(struct task_struct *tsk)
static void do_cgroup_task_dead(struct task_struct *tsk)
{
struct css_set *cset;
unsigned long flags;
@@ -7016,6 +7018,57 @@ void cgroup_task_dead(struct task_struct *tsk)
spin_unlock_irqrestore(&css_set_lock, flags);
}
#ifdef CONFIG_PREEMPT_RT
/*
* cgroup_task_dead() is called from finish_task_switch() which doesn't allow
* scheduling even in RT. As the task_dead path requires grabbing css_set_lock,
* this lead to sleeping in the invalid context warning bug. css_set_lock is too
* big to become a raw_spinlock. The task_dead path doesn't need to run
* synchronously but can't be delayed indefinitely either as the dead task pins
* the cgroup and task_struct can be pinned indefinitely. Bounce through lazy
* irq_work to allow batching while ensuring timely completion.
*/
static DEFINE_PER_CPU(struct llist_head, cgrp_dead_tasks);
static DEFINE_PER_CPU(struct irq_work, cgrp_dead_tasks_iwork);
static void cgrp_dead_tasks_iwork_fn(struct irq_work *iwork)
{
struct llist_node *lnode;
struct task_struct *task, *next;
lnode = llist_del_all(this_cpu_ptr(&cgrp_dead_tasks));
llist_for_each_entry_safe(task, next, lnode, cg_dead_lnode) {
do_cgroup_task_dead(task);
put_task_struct(task);
}
}
static void __init cgroup_rt_init(void)
{
int cpu;
for_each_possible_cpu(cpu) {
init_llist_head(per_cpu_ptr(&cgrp_dead_tasks, cpu));
per_cpu(cgrp_dead_tasks_iwork, cpu) =
IRQ_WORK_INIT_LAZY(cgrp_dead_tasks_iwork_fn);
}
}
void cgroup_task_dead(struct task_struct *task)
{
get_task_struct(task);
llist_add(&task->cg_dead_lnode, this_cpu_ptr(&cgrp_dead_tasks));
irq_work_queue(this_cpu_ptr(&cgrp_dead_tasks_iwork));
}
#else /* CONFIG_PREEMPT_RT */
static void __init cgroup_rt_init(void) {}
void cgroup_task_dead(struct task_struct *task)
{
do_cgroup_task_dead(task);
}
#endif /* CONFIG_PREEMPT_RT */
void cgroup_task_release(struct task_struct *task)
{
struct cgroup_subsys *ss;