Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing synchronous

On 03/04/24 09:38, Waiman Long wrote:

On 4/3/24 08:02, Michal Koutný wrote:

...

On Tue, Apr 02, 2024 at 11:30:11AM -0400, Waiman Long longman@redhat.com wrote:

...

Yes, there is a potential that a cpus_read_lock() may be called leading to deadlock. So unless we reverse the current cgroup_mutex --> cpu_hotplug_lock ordering, it is not safe to call cgroup_transfer_tasks() directly.

I see that cgroup_transfer_tasks() has the only user -- cpuset. What about bending it for the specific use like:

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h index 34aaf0e87def..64deb7212c5c 100644 --- a/include/linux/cgroup.h +++ b/include/linux/cgroup.h @@ -109,7 +109,7 @@ struct cgroup *cgroup_get_from_fd(int fd); struct cgroup *cgroup_v1v2_get_from_fd(int fd);

int cgroup_attach_task_all(struct task_struct *from, struct task_struct *); -int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from); +int cgroup_transfer_tasks_locked(struct cgroup *to, struct cgroup *from);

int cgroup_add_dfl_cftypes(struct cgroup_subsys *ss, struct cftype *cfts); int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts); diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c index 520a11cb12f4..f97025858c7a 100644 --- a/kernel/cgroup/cgroup-v1.c +++ b/kernel/cgroup/cgroup-v1.c @@ -91,7 +91,8 @@ EXPORT_SYMBOL_GPL(cgroup_attach_task_all);

• Return: %0 on success or a negative errno code on failure

*/ -int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from) +int cgroup_transfer_tasks_locked(struct cgroup *to, struct cgroup *from) { DEFINE_CGROUP_MGCTX(mgctx); struct cgrp_cset_link *link; @@ -106,9 +106,11 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from) if (ret) return ret;

• cgroup_lock();
• cgroup_attach_lock(true);

• /* The locking rules serve specific purpose of v1 cpuset hotplug

• * migration, see hotplug_update_tasks_legacy() and
  

• * cgroup_attach_lock() */
  

• lockdep_assert_held(&cgroup_mutex);

• lockdep_assert_cpus_held();

• percpu_down_write(&cgroup_threadgroup_rwsem);

  /* all tasks in @from are being moved, all csets are source */ spin_lock_irq(&css_set_lock);

@@ -144,8 +146,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from) } while (task && !ret); out_err: cgroup_migrate_finish(&mgctx);

• cgroup_attach_unlock(true);
• cgroup_unlock();

• percpu_up_write(&cgroup_threadgroup_rwsem); return ret; }

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index 13d27b17c889..94fb8b26f038 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -4331,7 +4331,7 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs) nodes_empty(parent->mems_allowed)) parent = parent_cs(parent);

• if (cgroup_transfer_tasks(parent->css.cgroup, cs->css.cgroup)) {

• if (cgroup_transfer_tasks_locked(parent->css.cgroup, cs->css.cgroup)) { pr_err("cpuset: failed to transfer tasks out of empty cpuset "); pr_cont_cgroup_name(cs->css.cgroup); pr_cont("\n");

@@ -4376,21 +4376,9 @@ hotplug_update_tasks_legacy(struct cpuset *cs,

 /*
  * Move tasks to the nearest ancestor with execution resources,

• * This is full cgroup operation which will also call back into
  

• * cpuset. Execute it asynchronously using workqueue.
  */
  

• if (is_empty && css_tryget_online(&cs->css)) {

• struct cpuset_remove_tasks_struct *s;
  

• s = kzalloc(sizeof(*s), GFP_KERNEL);
  

• if (WARN_ON_ONCE(!s)) {
  

• 	css_put(&cs->css);
  

• 	return;
  

• }
  

• s->cs = cs;
  

• INIT_WORK(&s->work, cpuset_migrate_tasks_workfn);
  

• schedule_work(&s->work);
  

• if (is_empty)

• remove_tasks_in_empty_cpuset(cs);
  

  } }

It still won't work because of the possibility of mutiple tasks involving in a circular locking dependency. The hotplug thread always acquire the cpu_hotplug_lock first before acquiring cpuset_mutex or cgroup_mtuex in this case (cpu_hotplug_lock --> cgroup_mutex). Other tasks calling into cgroup code will acquire the pair in the order cgroup_mutex --> cpu_hotplug_lock. This may lead to a deadlock if these 2 locking sequences happen at the same time. Lockdep will certainly spill out a splat because of this.

So unless we change all the relevant cgroup code to the new cpu_hotplug_lock --> cgroup_mutex locking order, the hotplug code can't call cgroup_transfer_tasks() directly.

IIUC that was Thomas' suggestion [1], but I can't tell yet how bad it would be to change cgroup_lock() to also do a cpus_read_lock().

Also, I gave Michal's patch a try and it looks like it's introducing a cgroup_threadgroup_rwsem -> cpuset_mutex ordering from cgroup_transfer_tasks_locked() `\ percpu_down_write(&cgroup_threadgroup_rwsem); cgroup_migrate() `\ cgroup_migrate_execute() `\ ss->can_attach() // cpuset_can_attach() `\ mutex_lock(&cpuset_mutex);

which is invalid, see below.

[1]: https://lore.kernel.org/lkml/87cyrfe7a3.ffs@tglx/

[ 77.627915] WARNING: possible circular locking dependency detected [ 77.628419] 6.9.0-rc1-00042-g844178b616c7-dirty #23 Tainted: G W [ 77.629035] ------------------------------------------------------ [ 77.629548] cpuhp/2/24 is trying to acquire lock: [ 77.629946] ffffffff82d680b0 (cgroup_threadgroup_rwsem){++++}-{0:0}, at: cgroup_transfer_tasks_locked+0x123/0x450 [ 77.630851] [ 77.630851] but task is already holding lock: [ 77.631397] ffffffff82d6c088 (cpuset_mutex){+.+.}-{3:3}, at: cpuset_update_active_cpus+0x352/0xca0 [ 77.632169] [ 77.632169] which lock already depends on the new lock. [ 77.632169] [ 77.632891] [ 77.632891] the existing dependency chain (in reverse order) is: [ 77.633521] [ 77.633521] -> #1 (cpuset_mutex){+.+.}-{3:3}: [ 77.634028] lock_acquire+0xc0/0x2d0 [ 77.634393] __mutex_lock+0xaa/0x710 [ 77.634751] cpuset_can_attach+0x6d/0x2c0 [ 77.635146] cgroup_migrate_execute+0x6f/0x520 [ 77.635565] cgroup_attach_task+0x2e2/0x450 [ 77.635989] __cgroup1_procs_write.isra.0+0xfd/0x150 [ 77.636440] kernfs_fop_write_iter+0x127/0x1c0 [ 77.636917] vfs_write+0x2b0/0x540 [ 77.637330] ksys_write+0x70/0xf0 [ 77.637707] int80_emulation+0xf8/0x1b0 [ 77.638183] asm_int80_emulation+0x1a/0x20 [ 77.638636] [ 77.638636] -> #0 (cgroup_threadgroup_rwsem){++++}-{0:0}: [ 77.639321] check_prev_add+0xeb/0xb20 [ 77.639751] __lock_acquire+0x12ac/0x16d0 [ 77.640345] lock_acquire+0xc0/0x2d0 [ 77.640903] percpu_down_write+0x33/0x260 [ 77.641347] cgroup_transfer_tasks_locked+0x123/0x450 [ 77.641826] cpuset_update_active_cpus+0x782/0xca0 [ 77.642268] sched_cpu_deactivate+0x1ad/0x1d0 [ 77.642677] cpuhp_invoke_callback+0x16b/0x6b0 [ 77.643098] cpuhp_thread_fun+0x1ba/0x240 [ 77.643488] smpboot_thread_fn+0xd8/0x1d0 [ 77.643873] kthread+0xce/0x100 [ 77.644209] ret_from_fork+0x2f/0x50 [ 77.644626] ret_from_fork_asm+0x1a/0x30 [ 77.645084] [ 77.645084] other info that might help us debug this: [ 77.645084] [ 77.645829] Possible unsafe locking scenario: [ 77.645829] [ 77.646356] CPU0 CPU1 [ 77.646748] ---- ---- [ 77.647143] lock(cpuset_mutex); [ 77.647529] lock(cgroup_threadgroup_rwsem); [ 77.648193] lock(cpuset_mutex); [ 77.648767] lock(cgroup_threadgroup_rwsem); [ 77.649216] [ 77.649216] *** DEADLOCK ***