Messages in this thread | | | Date | Wed, 3 Apr 2024 10:47:33 -0400 | Subject | Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing synchronous | From | Waiman Long <> |
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On 4/3/24 10:26, Valentin Schneider wrote: > 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().
Changing the locking order is certainly doable. I have taken a cursory look at it and at least the following files need to be changed:
kernel/bpf/cgroup.c kernel/cgroup/cgroup.c kernel/cgroup/legacy_freezer.c mm/memcontrol.c
That requires a lot more testing to make sure that there won't be a regression. Given that the call to cgroup_transfer_tasks() should be rare these days as it will only apply in the case of cgroup v1 under certain condtion, I am not sure this requirement justifies making such extensive changes. So I kind of defer it until we reach a consensus that it is the right thing to do.
Cheers, Longman
> > 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 *** >
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