Re: Bisected: Kernel deadlock bug related to cgroups.
From: Oleg Nesterov
Date: Wed Aug 31 2016 - 06:17:24 EST
On 08/30, Brent Lovelace wrote:
>
> I found a kernel deadlock regression bug introduced in the 4.4 kernel.
...
> I bisected to this commit id:
> ----------------------------------------------------------------------------------
> commit c9e75f0492b248aeaa7af8991a6fc9a21506bc96
> Author: Oleg Nesterov <oleg@xxxxxxxxxx>
> Date: Fri Nov 27 19:57:19 2015 +0100
>
> cgroup: pids: fix race between cgroup_post_fork() and cgroup_migrate()
Thanks Brent!
> systemd D ffff88007dfcfd10 0 1 0 0x00000000
> ffff88007dfcfd10 00ff88007dfcfcd8 0000000000000001 ffff88007e296f80
> ffff88007dff8000 ffff88007dfd0000 ffffffff82bae220 ffff880036758c00
> fffffffffffffff2 ffff88005e327b00 ffff88007dfcfd28 ffffffff816c571f
> Call Trace:
> [<ffffffff816c571f>] schedule+0x7a/0x8f
> [<ffffffff81126ed2>] percpu_down_write+0xad/0xc4
> [<ffffffff811220c8>] ? wake_up_atomic_t+0x25/0x25
> [<ffffffff81169b4d>] __cgroup_procs_write+0x72/0x229
> [<ffffffff8112b2eb>] ? lock_acquire+0x103/0x18f
so it sleeps in wait_event() waiting for active readers, and the new
readers will block. In particular, do_exit() will block.
> kworker/u8:2 D ffff880036befb58 0 185 2 0x00000000
> Workqueue: netns cleanup_net
> ffff880036befb58 00ff880036befbd0 ffffffff00000002 ffff88007e316f80
> ffff8800783e8000 ffff880036bf0000 ffff88005917bed0 ffff8800783e8000
> ffffffff816c8953 ffff88005917bed8 ffff880036befb70 ffffffff816c571f
> Call Trace:
> [<ffffffff816c8953>] ? usleep_range+0x3a/0x3a
> [<ffffffff816c571f>] schedule+0x7a/0x8f
> [<ffffffff816c8982>] schedule_timeout+0x2f/0xd8
> [<ffffffff816c9204>] ? _raw_spin_unlock_irq+0x27/0x3f
> [<ffffffff816c8953>] ? usleep_range+0x3a/0x3a
> [<ffffffff81129ea2>] ? trace_hardirqs_on_caller+0x16f/0x18b
> [<ffffffff816c5f44>] do_wait_for_common+0xf0/0x127
> [<ffffffff816c5f44>] ? do_wait_for_common+0xf0/0x127
> [<ffffffff811101c3>] ? wake_up_q+0x42/0x42
> [<ffffffff816c5ff2>] wait_for_common+0x36/0x50
> [<ffffffff816c6024>] wait_for_completion+0x18/0x1a
> [<ffffffff81106b5f>] kthread_stop+0xc8/0x217
> [<ffffffffa06d440c>] pg_net_exit+0xbc/0x112 [pktgen]
> [<ffffffff81604fa2>] ops_exit_list+0x3d/0x4e
> [<ffffffff81606123>] cleanup_net+0x19f/0x234
> [<ffffffff81101aa3>] process_one_work+0x237/0x46b
> [<ffffffff8110216d>] worker_thread+0x1e7/0x292
> [<ffffffff81101f86>] ? rescuer_thread+0x285/0x285
> [<ffffffff81106ded>] kthread+0xc4/0xcc
> [<ffffffff81106d29>] ? kthread_parkme+0x1f/0x1f
> [<ffffffff816c9d6f>] ret_from_fork+0x3f/0x70
> [<ffffffff81106d29>] ? kthread_parkme+0x1f/0x1f
> 3 locks held by kworker/u8:2/185:
> #0: ("%s""netns"){.+.+.+}, at: [<ffffffff811019ad>]
> process_one_work+0x141/0x46b
> #1: (net_cleanup_work){+.+.+.}, at: [<ffffffff811019ad>]
> process_one_work+0x141/0x46b
> #2: (net_mutex){+.+.+.}, at: [<ffffffff81605ffe>] cleanup_net+0x7a/0x234
Note that it sleeps with net_mutex held. Probably waiting for kpktgend_*
below.
> vsftpd D ffff880054867c68 0 4352 2611 0x00000000
> ffff880054867c68 00ff88005933a480 ffff880000000000 ffff88007e216f80
> ffff88005933a480 ffff880054868000 0000000000000246 ffff880054867cc0
> ffff88005933a480 ffffffff81cea268 ffff880054867c80 ffffffff816c571f
> Call Trace:
> [<ffffffff816c571f>] schedule+0x7a/0x8f
> [<ffffffff816c59b1>] schedule_preempt_disabled+0x10/0x19
> [<ffffffff816c64c0>] mutex_lock_nested+0x1c0/0x3a0
> [<ffffffff81606233>] ? copy_net_ns+0x7b/0xf8
> [<ffffffff81606233>] copy_net_ns+0x7b/0xf8
> [<ffffffff81606233>] ? copy_net_ns+0x7b/0xf8
> [<ffffffff811073c9>] create_new_namespaces+0xfc/0x16b
> [<ffffffff8110759c>] copy_namespaces+0x164/0x186
> [<ffffffff810ea6b1>] copy_process+0x10d2/0x195d
> [<ffffffff810eb094>] _do_fork+0x8c/0x2fb
> [<ffffffff81003044>] ? lockdep_sys_exit_thunk+0x12/0x14
> [<ffffffff810eb375>] SyS_clone+0x14/0x16
> [<ffffffff816c99b6>] entry_SYSCALL_64_fastpath+0x16/0x76
> 2 locks held by vsftpd/4352:
> #0: (&cgroup_threadgroup_rwsem){++++++}, at: [<ffffffff810e9b97>]
> copy_process+0x5b8/0x195d
> #1: (net_mutex){+.+.+.}, at: [<ffffffff81606233>] copy_net_ns+0x7b/0xf8
This waits for net_mutex held by kworker/u8:2 above. And with
cgroup_threadgroup_rwsem acquired for reading, that is why systemd
above hangs.
> kpktgend_0 D ffff88005917bce8 0 4354 2 0x00000000
> ffff88005917bce8 00ffffffa06d5d06 ffff880000000000 ffff88007e216f80
> ffff88007a4ec900 ffff88005917c000 ffff88007a4ec900 ffffffffa06d5d06
> ffff88005917bed0 0000000000000000 ffff88005917bd00 ffffffff816c571f
> Call Trace:
> [<ffffffffa06d5d06>] ? pg_net_init+0x346/0x346 [pktgen]
> [<ffffffff816c571f>] schedule+0x7a/0x8f
> [<ffffffff816c85ee>] rwsem_down_read_failed+0xdc/0xf8
> [<ffffffff8136d634>] call_rwsem_down_read_failed+0x14/0x30
> [<ffffffff8136d634>] ? call_rwsem_down_read_failed+0x14/0x30
> [<ffffffff810f91e0>] ? exit_signals+0x17/0x103
> [<ffffffff81126dc4>] ? percpu_down_read+0x4d/0x5f
> [<ffffffff810f91e0>] exit_signals+0x17/0x103
> [<ffffffff810ed342>] do_exit+0x105/0x9a4
> [<ffffffffa06d5d06>] ? pg_net_init+0x346/0x346 [pktgen]
> [<ffffffff81106df5>] kthread+0xcc/0xcc
> [<ffffffff81106d29>] ? kthread_parkme+0x1f/0x1f
> [<ffffffff816c9d6f>] ret_from_fork+0x3f/0x70
> [<ffffffff81106d29>] ? kthread_parkme+0x1f/0x1f
> 1 lock held by kpktgend_0/4354:
> #0: (&cgroup_threadgroup_rwsem){++++++}, at: [<ffffffff810f91e0>]
it can't take cgroup_threadgroup_rwsem for reading, so it can't exit,
and that is why kworker/u8:2 hangs.
> kpktgend_1 D ffff88007a4e3ce8 0 4355 2 0x00000000
...
> kpktgend_2 D ffff8800549f7ce8 0 4356 2 0x00000000
...
> kpktgend_3 D ffff88005e2b7ce8 0 4357 2 0x00000000
...
The same.
Could you try the recent 568ac888215c7fb2fab "cgroup: reduce read
locked section of cgroup_threadgroup_rwsem during fork" patch?
Attached below.
With this patch copy_net_ns() should be called outside of
cgroup_threadgroup_rwsem, the deadlock should hopefully go away.
Thanks,
Oleg.
---
commit 568ac888215c7fb2fabe8ea739b00ec3c1f5d440
Author: Balbir Singh <bsingharora@xxxxxxxxx>
Date: Wed Aug 10 15:43:06 2016 -0400
cgroup: reduce read locked section of cgroup_threadgroup_rwsem during fork
cgroup_threadgroup_rwsem is acquired in read mode during process exit
and fork. It is also grabbed in write mode during
__cgroups_proc_write(). I've recently run into a scenario with lots
of memory pressure and OOM and I am beginning to see
systemd
__switch_to+0x1f8/0x350
__schedule+0x30c/0x990
schedule+0x48/0xc0
percpu_down_write+0x114/0x170
__cgroup_procs_write.isra.12+0xb8/0x3c0
cgroup_file_write+0x74/0x1a0
kernfs_fop_write+0x188/0x200
__vfs_write+0x6c/0xe0
vfs_write+0xc0/0x230
SyS_write+0x6c/0x110
system_call+0x38/0xb4
This thread is waiting on the reader of cgroup_threadgroup_rwsem to
exit. The reader itself is under memory pressure and has gone into
reclaim after fork. There are times the reader also ends up waiting on
oom_lock as well.
__switch_to+0x1f8/0x350
__schedule+0x30c/0x990
schedule+0x48/0xc0
jbd2_log_wait_commit+0xd4/0x180
ext4_evict_inode+0x88/0x5c0
evict+0xf8/0x2a0
dispose_list+0x50/0x80
prune_icache_sb+0x6c/0x90
super_cache_scan+0x190/0x210
shrink_slab.part.15+0x22c/0x4c0
shrink_zone+0x288/0x3c0
do_try_to_free_pages+0x1dc/0x590
try_to_free_pages+0xdc/0x260
__alloc_pages_nodemask+0x72c/0xc90
alloc_pages_current+0xb4/0x1a0
page_table_alloc+0xc0/0x170
__pte_alloc+0x58/0x1f0
copy_page_range+0x4ec/0x950
copy_process.isra.5+0x15a0/0x1870
_do_fork+0xa8/0x4b0
ppc_clone+0x8/0xc
In the meanwhile, all processes exiting/forking are blocked almost
stalling the system.
This patch moves the threadgroup_change_begin from before
cgroup_fork() to just before cgroup_canfork(). There is no nee to
worry about threadgroup changes till the task is actually added to the
threadgroup. This avoids having to call reclaim with
cgroup_threadgroup_rwsem held.
tj: Subject and description edits.
Signed-off-by: Balbir Singh <bsingharora@xxxxxxxxx>
Acked-by: Zefan Li <lizefan@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v4.2+
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
diff --git a/kernel/fork.c b/kernel/fork.c
index 52e725d..aaf7823 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1404,7 +1404,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
p->real_start_time = ktime_get_boot_ns();
p->io_context = NULL;
p->audit_context = NULL;
- threadgroup_change_begin(current);
cgroup_fork(p);
#ifdef CONFIG_NUMA
p->mempolicy = mpol_dup(p->mempolicy);
@@ -1556,6 +1555,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
INIT_LIST_HEAD(&p->thread_group);
p->task_works = NULL;
+ threadgroup_change_begin(current);
/*
* Ensure that the cgroup subsystem policies allow the new process to be
* forked. It should be noted the the new process's css_set can be changed
@@ -1656,6 +1656,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
bad_fork_cancel_cgroup:
cgroup_cancel_fork(p);
bad_fork_free_pid:
+ threadgroup_change_end(current);
if (pid != &init_struct_pid)
free_pid(pid);
bad_fork_cleanup_thread:
@@ -1688,7 +1689,6 @@ bad_fork_cleanup_policy:
mpol_put(p->mempolicy);
bad_fork_cleanup_threadgroup_lock:
#endif
- threadgroup_change_end(current);
delayacct_tsk_free(p);
bad_fork_cleanup_count:
atomic_dec(&p->cred->user->processes);