[Devel] [RESEND PATCH vz10 1/2] ve/kthread: fix race when work can be added to stopped kthread worker #VSTOR-106887
Konstantin Khorenko
khorenko at virtuozzo.com
Tue Oct 28 19:15:02 MSK 2025
And where is the original patch mail?
i don't see it in the list:
https://lists.openvz.org/pipermail/devel/2025-October/date.html
--
Best regards,
Konstantin Khorenko,
Virtuozzo Linux Kernel Team
On 10/21/25 12:25, Pavel Tikhomirov wrote:
> Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
>
> (for both patches, will merge it tomorrow)
>
> On 10/21/25 17:18, Aleksei Oladko wrote:
>> This patch reintroduces the feature from commit 6d43ed1, which was reverted
>> due to a scenario where a hanging user-mode-helper in one container could
>> block the startup of other containers.
>>
>> Race can be reproduced with steps below:
>>
>> 1) Add these test patch to increase the race probability:
>>
>> kernel/umh.c
>> @@ -455,6 +456,8 @@ int call_usermodehelper_exec_ve(struct ve_struct *ve,
>> sub_info->queue = call_usermodehelper_queue_ve;
>> kthread_init_work(&sub_info->ve_work,
>> call_usermodehelper_exec_work_ve);
>> + while (VE_IS_RUNNING(ve))
>> + cond_resched();
>> } else {
>> sub_info->queue = call_usermodehelper_queue;
>> INIT_WORK(&sub_info->work, call_usermodehelper_exec_work);
>>
>> 2) Set corepattern to type pipe in CT:
>>
>> echo "|/bin/dd of=/vz/coredumps/core-%e-%t.%p" > /proc/sys/kernel/core_pattern
>>
>> 3) Generate high CPU load on all cores using tools like stress-ng
>>
>> stress-ng --futex $(nproc) --timeout 60s
>>
>> 4) Produce a segfault inside a container and next try to stop the
>> container killing init.
>>
>> Coredump creates "dd" work and ads it to ve_umh_worker, which is already
>> stopped and will never handle these work, and our work will hang
>> forever, and container will never stop:
>>
>> [<0>] call_usermodehelper_exec+0x168/0x1a0
>> [<0>] call_usermodehelper_exec_ve+0x96/0xe0
>> [<0>] do_coredump+0x60f/0xf40
>> [<0>] get_signal+0x834/0x960
>> [<0>] arch_do_signal_or_restart+0x29/0xf0
>> [<0>] irqentry_exit_to_user_mode+0x12e/0x1a0
>> [<0>] asm_exc_page_fault+0x26/0x30
>>
>> Fix is:
>>
>> 1) Before calling call_usermodehelper_exec for a ve, check that
>> the ve is running before adding work.
>>
>> 2) Add separate hepler counters for each ve.
>>
>> 3) In ve_stop_ns, after setting the VE_STATE_STOPPING state,
>> wait for the running helpers count to reach 0 before stopping umh.
>>
>> There are 2 cases:
>>
>> If the call_usermodehelper_exec_ve thread reaches call_usermodehelper_exec
>> after ve_stop_ns started wait_khelpers, it will already see that
>> the ve is no in the running state and will no queue the work.
>>
>> If the call_usermodehelper_exec_ve thread reaches call_usermodehelper_exec
>> before the VE_STATE_STOPPING state is set for the ve, then ve_stop_ns will
>> wait in wait_khelpers until all works have been queued. After that all
>> queued works will be processed in kthread_flush_worker.
>>
>> https://virtuozzo.atlassian.net/browse/VSTOR-106887
>>
>> Signed-off-by: Aleksei Oladko <aleksey.oladko at virtuozzo.com>
>> ---
>> include/linux/umh.h | 2 ++
>> include/linux/ve.h | 2 ++
>> kernel/umh.c | 27 ++++++++++++++++++++++++++-
>> kernel/ve/ve.c | 5 +++++
>> 4 files changed, 35 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/umh.h b/include/linux/umh.h
>> index 5647f1e64e39..35fc4023df74 100644
>> --- a/include/linux/umh.h
>> +++ b/include/linux/umh.h
>> @@ -56,6 +56,8 @@ extern int
>> call_usermodehelper_exec_ve(struct ve_struct *ve,
>> struct subprocess_info *info, int wait);
>>
>> +extern void wait_khelpers(struct ve_struct *ve);
>> +
>> #else /* CONFIG_VE */
>>
>> #define call_usermodehelper_ve(ve, ...) \
>> diff --git a/include/linux/ve.h b/include/linux/ve.h
>> index e944132f972f..37562dff25aa 100644
>> --- a/include/linux/ve.h
>> +++ b/include/linux/ve.h
>> @@ -93,6 +93,8 @@ struct ve_struct {
>> struct kthread_worker *kthreadd_worker;
>> struct task_struct *kthreadd_task;
>>
>> + atomic_t umh_running_helpers;
>> + struct wait_queue_head umh_helpers_waitq;
>> struct kthread_worker umh_worker;
>> struct task_struct *umh_task;
>>
>> diff --git a/kernel/umh.c b/kernel/umh.c
>> index bd49c108eb90..699403efe382 100644
>> --- a/kernel/umh.c
>> +++ b/kernel/umh.c
>> @@ -447,9 +447,30 @@ static void call_usermodehelper_exec_work_ve(struct kthread_work *work)
>> }
>> }
>>
>> +static void umh_running_helpers_inc(struct ve_struct *ve)
>> +{
>> + atomic_inc(&ve->umh_running_helpers);
>> + smp_mb__after_atomic();
>> +}
>> +
>> +static void umh_running_helpers_dec(struct ve_struct *ve)
>> +{
>> + if (atomic_dec_and_test(&ve->umh_running_helpers))
>> + wake_up(&ve->umh_helpers_waitq);
>> +}
>> +
>> +void wait_khelpers(struct ve_struct *ve)
>> +{
>> + wait_event(ve->umh_helpers_waitq,
>> + atomic_read(&ve->umh_running_helpers) == 0);
>> +}
>> +EXPORT_SYMBOL(wait_khelpers);
>> +
>> int call_usermodehelper_exec_ve(struct ve_struct *ve,
>> struct subprocess_info *sub_info, int wait)
>> {
>> + int ret = -EBUSY;
>> +
>> if (!ve_is_super(ve)) {
>> sub_info->ve = ve;
>> sub_info->queue = call_usermodehelper_queue_ve;
>> @@ -460,7 +481,11 @@ int call_usermodehelper_exec_ve(struct ve_struct *ve,
>> INIT_WORK(&sub_info->work, call_usermodehelper_exec_work);
>> }
>>
>> - return call_usermodehelper_exec(sub_info, wait);
>> + umh_running_helpers_inc(ve);
>> + if (VE_IS_RUNNING(ve))
>> + ret = call_usermodehelper_exec(sub_info, wait);
>> + umh_running_helpers_dec(ve);
>> + return ret;
>> }
>> EXPORT_SYMBOL(call_usermodehelper_exec_ve);
>>
>> diff --git a/kernel/ve/ve.c b/kernel/ve/ve.c
>> index cb77f7b7e4cd..d5dc15942ab5 100644
>> --- a/kernel/ve/ve.c
>> +++ b/kernel/ve/ve.c
>> @@ -88,6 +88,8 @@ struct ve_struct ve0 = {
>> .nd_neigh_nr = ATOMIC_INIT(0),
>> .mnt_nr = ATOMIC_INIT(0),
>> .meminfo_val = VE_MEMINFO_SYSTEM,
>> + .umh_running_helpers = ATOMIC_INIT(0),
>> + .umh_helpers_waitq = __WAIT_QUEUE_HEAD_INITIALIZER(ve0.umh_helpers_waitq),
>> .vdso_64 = (struct vdso_image*)&vdso_image_64,
>> .vdso_32 = (struct vdso_image*)&vdso_image_32,
>> .release_list_lock = __SPIN_LOCK_UNLOCKED(
>> @@ -480,6 +482,8 @@ static int ve_start_umh(struct ve_struct *ve)
>> {
>> struct task_struct *task;
>>
>> + atomic_set(&ve->umh_running_helpers, 0);
>> + init_waitqueue_head(&ve->umh_helpers_waitq);
>> kthread_init_worker(&ve->umh_worker);
>>
>> task = kthread_create_on_node_ve_flags(ve, 0, kthread_worker_fn,
>> @@ -814,6 +818,7 @@ void ve_stop_ns(struct pid_namespace *pid_ns)
>> */
>> up_write(&ve->op_sem);
>>
>> + wait_khelpers(ve);
>> /*
>> * release_agent works on top of umh_worker, so we must make sure, that
>> * ve workqueue is stopped first.
>
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