Re: [PATCH] sched/schedutil: Fix deadlock between cpuset and cpu hotplug when using schedutil
From: Qais Yousef
Date: Fri Jul 29 2022 - 04:40:25 EST
On 07/29/22 15:33, Xuewen Yan wrote:
> Hi Tejun
>
> On Thu, Jul 28, 2022 at 4:09 AM Tejun Heo <tj@xxxxxxxxxx> wrote:
> >
> > Hello,
> >
> >
> > Can youp please see whether the following patch fixes the problem?
> >
>
> I have tested the patch, sadly, although the original deadlock call
> stack is gone, there seems to be a new problem.
> I did a preliminary analysis, but no further in-depth analysis. I
> haven't found the root cause yet, but I think I should let you know
> the test results first.
> Once I find the root cause, I'll report back immediately.
> The thread-A is waiting for the cpu_hotplug_lock's writer, but the
> writer is waiting for the readers:
I *think* it's because we haven't removed cpus_read_lock() from
cpuset_attach(). So we end up holding the lock twice in the same path. Since we
hold it unconditionally now, we should remove cpuset dependency on
cpus_read_lock() I believe.
Cheers
--
Qais Yousef
>
> PID: 1106 TASK: ffffff8097e90000 CPU: 7 COMMAND: "OomAdjuster"
> #0 [ffffffc011a23850] __switch_to at ffffffc0081229b4
> #1 [ffffffc011a238b0] __schedule at ffffffc009c824f8
> #2 [ffffffc011a23910] schedule at ffffffc009c82b50
> #3 [ffffffc011a23970] percpu_rwsem_wait at ffffffc00828fbf4
> #4 [ffffffc011a239b0] __percpu_down_read at ffffffc0082901a8
> #5 [ffffffc011a239e0] cpus_read_lock at ffffffc0081dadc8
> #6 [ffffffc011a23a20] cpuset_attach at ffffffc008366f10
> #7 [ffffffc011a23a90] cgroup_migrate_execute at ffffffc00834f808
> #8 [ffffffc011a23b60] cgroup_attach_task at ffffffc0083539f0
> #9 [ffffffc011a23bd0] __cgroup1_procs_write at ffffffc00835f6e8
> #10 [ffffffc011a23c30] cgroup1_tasks_write at ffffffc00835f054
> #11 [ffffffc011a23c60] cgroup_file_write at ffffffc008348b2c
> #12 [ffffffc011a23c90] kernfs_fop_write_iter at ffffffc008754514
> #13 [ffffffc011a23d50] vfs_write at ffffffc008607ac8
> #14 [ffffffc011a23db0] ksys_write at ffffffc008607714
> #15 [ffffffc011a23df0] __arm64_sys_write at ffffffc00860767c
> #16 [ffffffc011a23e10] invoke_syscall at ffffffc00813dcac
> #17 [ffffffc011a23e30] el0_svc_common at ffffffc00813dbc4
> #18 [ffffffc011a23e70] do_el0_svc at ffffffc00813daa8
> #19 [ffffffc011a23e80] el0_svc at ffffffc0098828d8
> #20 [ffffffc011a23ea0] el0t_64_sync_handler at ffffffc00988284c
> #21 [ffffffc011a23fe0] el0t_64_sync at ffffffc008091e44
>
> cpu_hotplug_lock = $3 = {
> rss = {
> gp_state = 2,
> gp_count = 1,
> gp_wait = {
> lock = {
> {
> rlock = {
> raw_lock = {
> {
> val = {
> counter = 0
> },
> {
> locked = 0 '\000',
> pending = 0 '\000'
> },
> {
> locked_pending = 0,
> tail = 0
> }
> }
> }
> }
> }
> },
> head = {
> next = 0xffffffc00b0eb6a0 <cpu_hotplug_lock+16>,
> prev = 0xffffffc00b0eb6a0 <cpu_hotplug_lock+16>
> }
> },
> cb_head = {
> next = 0x0,
> func = 0x0
> }
> },
> read_count = 0xffffffc00b0a0808 <__percpu_rwsem_rc_cpu_hotplug_lock>,
> writer = {
> task = 0xffffff80f303ca00
> },
> {
> waiters = {
> lock = {
> {
> rlock = {
> raw_lock = {
> {
> val = {
> counter = 0
> },
> {
> locked = 0 '\000',
> pending = 0 '\000'
> },
> {
> locked_pending = 0,
> tail = 0
> }
> }
> }
> }
> }
> },
> head = {
> next = 0xffffffc011a23958,
> prev = 0xffffffc01430bcb8
> }
> },
> destroy_list_entry = {
> next = 0x0,
> prev = 0xffffffc011a23958
> }
> },
> block = {
> counter = 1
> }
> }
>
> PID: 15760 TASK: ffffff80f303ca00 CPU: 5 COMMAND: "test.sh"
> #0 [ffffffc0129639a0] __switch_to at ffffffc0081229b4
> #1 [ffffffc012963a00] __schedule at ffffffc009c824f8
> #2 [ffffffc012963a60] schedule at ffffffc009c82b50
> #3 [ffffffc012963a90] percpu_down_write at ffffffc00828f9d8
> #4 [ffffffc012963ae0] _cpu_down at ffffffc009884550
> #5 [ffffffc012963b40] cpu_device_down at ffffffc0081df814
> #6 [ffffffc012963b60] cpu_subsys_offline at ffffffc008d8dd8c
> #7 [ffffffc012963b90] device_offline at ffffffc008d77124
> #8 [ffffffc012963bd0] online_store at ffffffc008d76d44
> #9 [ffffffc012963c30] dev_attr_store at ffffffc008d7ba30
> #10 [ffffffc012963c60] sysfs_kf_write at ffffffc0087578d0
> #11 [ffffffc012963c90] kernfs_fop_write_iter at ffffffc008754514
> #12 [ffffffc012963d50] vfs_write at ffffffc008607ac8
> #13 [ffffffc012963db0] ksys_write at ffffffc008607714
> #14 [ffffffc012963df0] __arm64_sys_write at ffffffc00860767c
> #15 [ffffffc012963e10] invoke_syscall at ffffffc00813dcac
> #16 [ffffffc012963e30] el0_svc_common at ffffffc00813dbf4
> #17 [ffffffc012963e70] do_el0_svc at ffffffc00813daa8
> #18 [ffffffc012963e80] el0_svc at ffffffc0098828d8
> #19 [ffffffc012963ea0] el0t_64_sync_handler at ffffffc00988284c
> #20 [ffffffc012963fe0] el0t_64_sync at ffffffc008091e44
>
> Thanks!
> BR
> --
> xuewen.yan
>
> > Thanks.
> >
> > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > index 13c8e91d78620..7caefc8af9127 100644
> > --- a/kernel/cgroup/cgroup.c
> > +++ b/kernel/cgroup/cgroup.c
> > @@ -2345,6 +2345,38 @@ int task_cgroup_path(struct task_struct *task, char *buf, size_t buflen)
> > }
> > EXPORT_SYMBOL_GPL(task_cgroup_path);
> >
> > +/**
> > + * lock_threadgroup - Stabilize threadgroups
> > + *
> > + * cgroup migration operations need the threadgroups stabilized against forks
> > + * and exits, which can be achieved by write-locking cgroup_threadgroup_rwsem.
> > + *
> > + * Note that write-locking threadgdroup_rwsem is nested inside CPU hotplug
> > + * disable. This is because cpuset needs CPU hotplug disabled during ->attach()
> > + * and bringing up a CPU may involve creating new tasks which can require
> > + * read-locking threadgroup_rwsem. If we call cpuset's ->attach() with
> > + * threadgroup_rwsem write-locked with hotplug enabled, we can deadlock by
> > + * cpuset waiting for an on-going CPU hotplug operation which in turn is waiting
> > + * for the threadgroup_rwsem to be released to create new tasks. See the
> > + * following for more details:
> > + *
> > + * http://lkml.kernel.org/r/20220711174629.uehfmqegcwn2lqzu@wubuntu
> > + */
> > +static void lock_threadgroup(void)
> > +{
> > + cpus_read_lock();
> > + percpu_down_write(&cgroup_threadgroup_rwsem);
> > +}
> > +
> > +/**
> > + * lock_threadgroup - Undo lock_threadgroup
> > + */
> > +static void unlock_threadgroup(void)
> > +{
> > + percpu_up_write(&cgroup_threadgroup_rwsem);
> > + cpus_read_unlock();
> > +}
> > +
> > /**
> > * cgroup_migrate_add_task - add a migration target task to a migration context
> > * @task: target task
> > @@ -2822,7 +2854,6 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
> >
> > struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
> > bool *locked)
> > - __acquires(&cgroup_threadgroup_rwsem)
> > {
> > struct task_struct *tsk;
> > pid_t pid;
> > @@ -2840,7 +2871,7 @@ struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
> > */
> > lockdep_assert_held(&cgroup_mutex);
> > if (pid || threadgroup) {
> > - percpu_down_write(&cgroup_threadgroup_rwsem);
> > + lock_threadgroup();
> > *locked = true;
> > } else {
> > *locked = false;
> > @@ -2876,7 +2907,7 @@ struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
> >
> > out_unlock_threadgroup:
> > if (*locked) {
> > - percpu_up_write(&cgroup_threadgroup_rwsem);
> > + unlock_threadgroup();
> > *locked = false;
> > }
> > out_unlock_rcu:
> > @@ -2885,7 +2916,6 @@ struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
> > }
> >
> > void cgroup_procs_write_finish(struct task_struct *task, bool locked)
> > - __releases(&cgroup_threadgroup_rwsem)
> > {
> > struct cgroup_subsys *ss;
> > int ssid;
> > @@ -2894,7 +2924,8 @@ void cgroup_procs_write_finish(struct task_struct *task, bool locked)
> > put_task_struct(task);
> >
> > if (locked)
> > - percpu_up_write(&cgroup_threadgroup_rwsem);
> > + unlock_threadgroup();
> > +
> > for_each_subsys(ss, ssid)
> > if (ss->post_attach)
> > ss->post_attach();
> > @@ -2953,7 +2984,7 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
> >
> > lockdep_assert_held(&cgroup_mutex);
> >
> > - percpu_down_write(&cgroup_threadgroup_rwsem);
> > + lock_threadgroup();
> >
> > /* look up all csses currently attached to @cgrp's subtree */
> > spin_lock_irq(&css_set_lock);
> > @@ -2984,7 +3015,7 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
> > ret = cgroup_migrate_execute(&mgctx);
> > out_finish:
> > cgroup_migrate_finish(&mgctx);
> > - percpu_up_write(&cgroup_threadgroup_rwsem);
> > + unlock_threadgroup();
> > return ret;
> > }
> >