Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing synchronous
From: Waiman Long
Date: Wed Apr 03 2024 - 10:48:52 EST
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@xxxxxxxxxx> 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 ***