[Devel] [PATCH vz10] cgroup: fix css_set_lock IRQ unsafe usage in check_freeze_timeout()

[Aleksei Oladko]

On 3/17/26 3:08 PM, Vasileios Almpanis wrote:
> lockdep reports an inconsistent lock state involving css_set_lock,
> triggered via cgroup_events_show() -> check_freeze_timeout():
>
>    WARNING: inconsistent lock state
>    inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
>
>    Possible unsafe locking scenario:
>
>          CPU0
>          ----
>     lock(css_set_lock);
>     <Interrupt>
>       lock(css_set_lock);
>
>     *** DEADLOCK ***
>
> The lock is currently taken with spin_lock()  in check_freeze_timeout(),
> which does not disable interrupts. However, css_set_lock is also
> acquired from IRQ/softirq context in RCU callbacks during task
> teardown, making it unsafe to take the lock with interrupts enabled.
>
> This can lead to a deadlock if an interrupt fires while holding
> css_set_lock and attempts to re-acquire it.
>
> Fix this by switching to spin_lock_irq() semantics via
> scoped_guard(spinlock_irq, ...), ensuring interrupts are disabled
> while holding css_set_lock in this path.
>
> This matches the locking requirements of css_set_lock.
>
> Fixes: 3cc887c3d0faa ("cgroup-v2/freezer: Print information about unfreezable process")
> https://virtuozzo.atlassian.net/browse/VSTOR-127046
>
> Signed-off-by: Vasileios Almpanis <vasileios.almpanis at virtuozzo.com>
>
> Feature: cgroup/freeze: enhance logging
> ---
>   kernel/cgroup/cgroup.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 040e7e36c238..376c9bbd1c41 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -4296,7 +4296,7 @@ static void check_freeze_timeout(struct cgroup *cgrp)
>   	u64 freeze_jiffies;
>   	int timeout;
>   
> -	scoped_guard(spinlock, &css_set_lock) {
> +	scoped_guard(spinlock_irq, &css_set_lock) {
Since the lock is only shared between process context and softirqs, 
spinlock_bh should be sufficient here. There is no need to disable irq.
>   		if (!test_bit(CGRP_FREEZE, &cgrp->flags) ||
>   		    test_bit(CGRP_FROZEN, &cgrp->flags))
>   			return;