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Date:	Mon, 26 Jan 2015 18:46:12 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Kirill Tkhai <ktkhai@...allels.com>, linux-kernel@...r.kernel.org
CC:	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Burke Libbey <burke.libbey@...pify.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: [PATCH] sched: Fix race between task_group and sched_task_group

On 10/27/2014 06:18 AM, Kirill Tkhai wrote:
> The race may happen when somebody is changing task_group of a forking task.
> Child's cgroup is the same as parent's after dup_task_struct() (there just
> memory copying). Also, cfs_rq and rt_rq are the same as parent's.
> 
> But if parent changes its task_group before it's called cgroup_post_fork(),
> we do not reflect this situation on child. Child's cfs_rq and rt_rq remain
> the same, while child's task_group changes in cgroup_post_fork().
> 
> To fix this we introduce fork() method, which calls sched_move_task() directly.
> This function changes sched_task_group on appropriate (also its logic has
> no problem with freshly created tasks, so we shouldn't introduce something
> special; we are able just to use it).
> 
> Possibly, this decides the Burke Libbey's problem: https://lkml.org/lkml/2014/10/24/456

Hi,

This seems to cause the following lockdep warning:

[ 3517.958378] ======================================================
[ 3517.959661] [ INFO: possible circular locking dependency detected ]
[ 3517.960172] 3.19.0-rc5-next-20150123-sasha-00063-gf82b1d7 #1824 Not tainted
[ 3517.960172] -------------------------------------------------------
[ 3517.960172] trinity-c7/29839 is trying to acquire lock:
[ 3517.960172] (&(&base->lock)->rlock){-.-.-.}, at: lock_timer_base.isra.19 (kernel/time/timer.c:751)
[ 3517.960172]
[ 3517.960172] but task is already holding lock:
[ 3517.960172] (&ctx->lock){-.-.-.}, at: perf_event_context_sched_in (kernel/events/core.c:2600)
[ 3517.960172]
[ 3517.960172] which lock already depends on the new lock.
[ 3517.960172]
[ 3517.960172]
[ 3517.960172] the existing dependency chain (in reverse order) is:
[ 3517.960172]
-> #5 (&ctx->lock){-.-.-.}:
[ 3517.960172] lock_acquire (kernel/locking/lockdep.c:3604)
[ 3517.960172] _raw_spin_lock (include/linux/spinlock_api_smp.h:145 kernel/locking/spinlock.c:151)
[ 3517.960172] __perf_event_task_sched_out (kernel/events/core.c:2434 kernel/events/core.c:2460)
[ 3517.960172] __schedule (include/linux/perf_event.h:730 kernel/sched/core.c:2209 kernel/sched/core.c:2333 kernel/sched/core.c:2823)
[ 3517.960172] schedule (kernel/sched/core.c:2853)
[ 3517.960172] p9_client_rpc (net/9p/client.c:756 (discriminator 13))
[ 3517.960172] p9_client_read (net/9p/client.c:1582)
[ 3517.960172] v9fs_fid_readn (fs/9p/vfs_file.c:386)
[ 3517.960172] v9fs_fid_readpage (fs/9p/vfs_addr.c:71)
[ 3517.960172] v9fs_vfs_readpage (fs/9p/vfs_addr.c:105)
[ 3517.960172] filemap_fault (mm/filemap.c:1763 mm/filemap.c:1944)
[ 3517.960172] __do_fault (mm/memory.c:2654)
[ 3517.960172] handle_mm_fault (mm/memory.c:2842 mm/memory.c:3143 mm/memory.c:3267 mm/memory.c:3296)
[ 3517.960172] __do_page_fault (arch/x86/mm/fault.c:1233)
[ 3517.960172] trace_do_page_fault (arch/x86/mm/fault.c:1325 include/linux/jump_label.h:114 include/linux/context_tracking_state.h:27 include/linux/context_tracking.h:45 arch/x86/mm/fault.c:1326)
[ 3517.960172] do_async_page_fault (arch/x86/kernel/kvm.c:280)
[ 3517.960172] async_page_fault (arch/x86/kernel/entry_64.S:1286)
[ 3517.960172]
-> #4 (&rq->lock){-.-.-.}:
[ 3517.960172] lock_acquire (kernel/locking/lockdep.c:3604)
[ 3517.960172] _raw_spin_lock (include/linux/spinlock_api_smp.h:145 kernel/locking/spinlock.c:151)
[ 3517.960172] task_rq_lock (kernel/sched/core.c:344)
[ 3517.960172] sched_move_task (kernel/sched/core.c:7556)
[ 3517.960172] cpu_cgroup_fork (kernel/sched/core.c:8003)
[ 3517.960172] cgroup_post_fork (kernel/cgroup.c:5239 (discriminator 2))
[ 3517.960172] copy_process (kernel/fork.c:1544)
[ 3517.960172] do_fork (kernel/fork.c:1653)
[ 3517.960172] kernel_thread (kernel/fork.c:1702)
[ 3517.960172] rest_init (init/main.c:406)
[ 3517.960172] start_kernel (init/main.c:503)
[ 3517.960172] x86_64_start_reservations (arch/x86/kernel/head64.c:199)
[ 3517.960172] x86_64_start_kernel (arch/x86/kernel/head64.c:188)
[ 3517.960172]
-> #3 (&p->pi_lock){-.-.-.}:
[ 3517.960172] lock_acquire (kernel/locking/lockdep.c:3604)
[ 3517.960172] _raw_spin_lock_irqsave (include/linux/spinlock_api_smp.h:119 kernel/locking/spinlock.c:159)
[ 3517.960172] try_to_wake_up (kernel/sched/core.c:1704)
[ 3517.960172] default_wake_function (kernel/sched/core.c:2995)
[ 3517.960172] autoremove_wake_function (kernel/sched/wait.c:295)
[ 3517.960172] __wake_up_common (kernel/sched/wait.c:73)
[ 3517.960172] __wake_up (include/linux/spinlock.h:372 kernel/sched/wait.c:96)
[ 3517.960172] wakeup_kswapd (mm/vmscan.c:3519)
[ 3517.960172] __alloc_pages_nodemask (mm/page_alloc.c:2530 mm/page_alloc.c:2628 mm/page_alloc.c:2849)
[ 3517.960172] new_page_node (mm/migrate.c:1180)
[ 3517.960172] migrate_pages (mm/migrate.c:913 mm/migrate.c:1112)
[ 3517.960172] SYSC_move_pages (mm/migrate.c:1265 mm/migrate.c:1340 mm/migrate.c:1495)
[ 3517.960172] SyS_move_pages (mm/migrate.c:1440)
[ 3517.960172] tracesys_phase2 (arch/x86/kernel/entry_64.S:530)
[ 3517.960172]
-> #2 (&pgdat->kswapd_wait){..-.-.}:
[ 3517.960172] lock_acquire (kernel/locking/lockdep.c:3604)
[ 3517.960172] _raw_spin_lock_irqsave (include/linux/spinlock_api_smp.h:119 kernel/locking/spinlock.c:159)
[ 3517.960172] __wake_up (kernel/sched/wait.c:95)
[ 3517.960172] wakeup_kswapd (mm/vmscan.c:3519)
[ 3517.960172] __alloc_pages_nodemask (mm/page_alloc.c:2530 mm/page_alloc.c:2628 mm/page_alloc.c:2849)
[ 3517.960172] alloc_pages_current (mm/mempolicy.c:2147)
[ 3517.960172] __get_free_pages (mm/page_alloc.c:2885)
[ 3517.960172] alloc_loc_track (mm/slub.c:3963)
[ 3517.960172] process_slab (mm/slub.c:4029 mm/slub.c:4063)
[ 3517.960172] list_locations (mm/slub.c:4095 (discriminator 3))
[ 3517.960172] alloc_calls_show (mm/slub.c:4700)
[ 3517.960172] slab_attr_show (mm/slub.c:4950)
[ 3517.960172] sysfs_kf_seq_show (fs/sysfs/file.c:64)
[ 3517.960172] kernfs_seq_show (fs/kernfs/file.c:169)
[ 3517.960172] seq_read (fs/seq_file.c:228)
[ 3517.960172] kernfs_fop_read (fs/kernfs/file.c:251)
[ 3517.960172] __vfs_read (fs/read_write.c:430)
[ 3517.960172] vfs_read (fs/read_write.c:446)
[ 3517.960172] kernel_read (fs/exec.c:819)
[ 3517.960172] copy_module_from_fd.isra.28 (kernel/module.c:2547)
[ 3517.960172] SyS_finit_module (kernel/module.c:3429 kernel/module.c:3413)
[ 3517.960172] tracesys_phase2 (arch/x86/kernel/entry_64.S:530)
[ 3517.960172]
-> #1 (&(&n->list_lock)->rlock){-.-.-.}:
[ 3517.960172] lock_acquire (kernel/locking/lockdep.c:3604)
[ 3517.960172] _raw_spin_lock (include/linux/spinlock_api_smp.h:145 kernel/locking/spinlock.c:151)
[ 3517.960172] get_partial_node.isra.35 (mm/slub.c:1630)
[ 3517.960172] __slab_alloc (mm/slub.c:1737 mm/slub.c:2207 mm/slub.c:2378)
[ 3517.960172] kmem_cache_alloc (mm/slub.c:2459 mm/slub.c:2506)
[ 3517.960172] __debug_object_init (include/linux/slab.h:572 lib/debugobjects.c:99 lib/debugobjects.c:312)
[ 3517.960172] debug_object_init (lib/debugobjects.c:365)
[ 3517.960172] timer_fixup_activate (kernel/time/timer.c:512)
[ 3517.960172] debug_object_activate (lib/debugobjects.c:280 lib/debugobjects.c:439)
[ 3517.960172] mod_timer (kernel/time/timer.c:589 include/linux/jump_label.h:114 include/trace/events/timer.h:44 kernel/time/timer.c:641 kernel/time/timer.c:778 kernel/time/timer.c:897)
[ 3517.960172] isicom_init (drivers/tty/isicom.c:1708)
[ 3517.960172] do_one_initcall (init/main.c:798)
[ 3517.960172] kernel_init_freeable (init/main.c:863 init/main.c:871 init/main.c:890 init/main.c:1011)
[ 3517.960172] kernel_init (init/main.c:943)
[ 3517.960172] ret_from_fork (arch/x86/kernel/entry_64.S:349)
[ 3517.960172]
-> #0 (&(&base->lock)->rlock){-.-.-.}:
[ 3517.960172] __lock_acquire (kernel/locking/lockdep.c:1842 kernel/locking/lockdep.c:1947 kernel/locking/lockdep.c:2133 kernel/locking/lockdep.c:3184)
[ 3517.960172] lock_acquire (kernel/locking/lockdep.c:3604)
[ 3517.960172] _raw_spin_lock_irqsave (include/linux/spinlock_api_smp.h:119 kernel/locking/spinlock.c:159)
[ 3517.960172] lock_timer_base.isra.19 (kernel/time/timer.c:751)
[ 3517.960172] mod_timer (kernel/time/timer.c:774 kernel/time/timer.c:897)
[ 3517.960172] add_timer (kernel/time/timer.c:947)
[ 3517.960172] __queue_delayed_work (kernel/workqueue.c:1452)
[ 3517.960172] queue_delayed_work_on (kernel/workqueue.c:1480)
[ 3517.960172] schedule_orphans_remove (kernel/events/core.c:1416)
[ 3517.960172] event_sched_in.isra.47 (kernel/events/core.c:1793)
[ 3517.960172] group_sched_in (kernel/events/core.c:1816)
[ 3517.960172] ctx_sched_in (kernel/events/core.c:2547 kernel/events/core.c:2578)
[ 3517.960172] perf_event_sched_in (kernel/events/core.c:1930)
[ 3517.960172] perf_event_context_sched_in (kernel/events/core.c:2613)
[ 3517.960172] __perf_event_task_sched_in (kernel/events/core.c:2703)
[ 3517.960172] finish_task_switch (include/linux/perf_event.h:721 kernel/sched/core.c:2257)
[ 3517.960172] __schedule (kernel/sched/core.c:2368 kernel/sched/core.c:2823)
[ 3517.960172] schedule_user (kernel/sched/core.c:2852 include/linux/jump_label.h:114 include/linux/context_tracking_state.h:27 include/linux/context_tracking.h:45 kernel/sched/core.c:2871)
[ 3517.960172] retint_careful (arch/x86/kernel/entry_64.S:905)
[ 3517.960172]
[ 3517.960172] other info that might help us debug this:
[ 3517.960172]
[ 3517.960172] Chain exists of:
&(&base->lock)->rlock --> &rq->lock --> &ctx->lock

[ 3517.960172]  Possible unsafe locking scenario:
[ 3517.960172]
[ 3517.960172]        CPU0                    CPU1
[ 3517.960172]        ----                    ----
[ 3517.960172]   lock(&ctx->lock);
[ 3517.960172]                                lock(&rq->lock);
[ 3517.960172]                                lock(&ctx->lock);
[ 3517.960172]   lock(&(&base->lock)->rlock);
[ 3517.960172]
[ 3517.960172]  *** DEADLOCK ***
[ 3517.960172]
[ 3517.960172] 2 locks held by trinity-c7/29839:
[ 3517.960172] #0: (&cpuctx_lock){-.-.-.}, at: perf_event_context_sched_in (kernel/events/core.c:340 kernel/events/core.c:2599)
[ 3517.960172] #1: (&ctx->lock){-.-.-.}, at: perf_event_context_sched_in (kernel/events/core.c:2600)
[ 3517.960172]
[ 3517.960172] stack backtrace:
[ 3517.960172] CPU: 7 PID: 29839 Comm: trinity-c7 Not tainted 3.19.0-rc5-next-20150123-sasha-00063-gf82b1d7 #1824
[ 3517.960172]  ffffffffa67fa820 00000000167dc919 ffff880353a8f700 ffffffffa0a859d2
[ 3517.960172]  0000000000000000 ffffffffa67fcef0 ffff880353a8f760 ffffffff96407cd9
[ 3517.960172]  ffff88017c680138 ffffffffa6879cf0 0000000000000000 ffff880353a90000
[ 3517.960172] Call Trace:
[ 3517.960172] dump_stack (lib/dump_stack.c:52)
[ 3517.960172] print_circular_bug (kernel/locking/lockdep.c:1217)
[ 3517.960172] __lock_acquire (kernel/locking/lockdep.c:1842 kernel/locking/lockdep.c:1947 kernel/locking/lockdep.c:2133 kernel/locking/lockdep.c:3184)
[ 3517.960172] ? debug_check_no_locks_freed (kernel/locking/lockdep.c:3051)
[ 3517.960172] ? __lock_acquire (kernel/locking/lockdep.c:3144)
[ 3517.960172] lock_acquire (kernel/locking/lockdep.c:3604)
[ 3517.960172] ? lock_timer_base.isra.19 (kernel/time/timer.c:751)
[ 3517.960172] _raw_spin_lock_irqsave (include/linux/spinlock_api_smp.h:119 kernel/locking/spinlock.c:159)
[ 3517.960172] ? lock_timer_base.isra.19 (kernel/time/timer.c:751)
[ 3517.960172] lock_timer_base.isra.19 (kernel/time/timer.c:751)
[ 3517.960172] mod_timer (kernel/time/timer.c:774 kernel/time/timer.c:897)
[ 3517.960172] ? x86_pmu_start_txn (arch/x86/kernel/cpu/perf_event.c:1024)
[ 3517.960172] ? msleep (kernel/time/timer.c:886)
[ 3517.960172] add_timer (kernel/time/timer.c:947)
[ 3517.960172] __queue_delayed_work (kernel/workqueue.c:1452)
[ 3517.960172] queue_delayed_work_on (kernel/workqueue.c:1480)
[ 3517.960172] schedule_orphans_remove (kernel/events/core.c:1416)
[ 3517.960172] event_sched_in.isra.47 (kernel/events/core.c:1793)
[ 3517.960172] ? sched_clock (./arch/x86/include/asm/paravirt.h:192 arch/x86/kernel/tsc.c:304)
[ 3517.960172] group_sched_in (kernel/events/core.c:1816)
[ 3517.960172] ? sched_clock_cpu (kernel/sched/clock.c:311)
[ 3517.960172] ctx_sched_in (kernel/events/core.c:2547 kernel/events/core.c:2578)
[ 3517.960172] ? perf_event_context_sched_in (kernel/events/core.c:2600)
[ 3517.960172] perf_event_sched_in (kernel/events/core.c:1930)
[ 3517.960172] perf_event_context_sched_in (kernel/events/core.c:2613)
[ 3517.960172] __perf_event_task_sched_in (kernel/events/core.c:2703)
[ 3517.960172] ? perf_pmu_enable (kernel/events/core.c:2694)
[ 3517.960172] finish_task_switch (include/linux/perf_event.h:721 kernel/sched/core.c:2257)
[ 3517.960172] __schedule (kernel/sched/core.c:2368 kernel/sched/core.c:2823)
[ 3517.960172] ? trace_hardirqs_on_caller (kernel/locking/lockdep.c:2554 kernel/locking/lockdep.c:2601)
[ 3517.960172] schedule_user (kernel/sched/core.c:2852 include/linux/jump_label.h:114 include/linux/context_tracking_state.h:27 include/linux/context_tracking.h:45 kernel/sched/core.c:2871)
[ 3517.960172] retint_careful (arch/x86/kernel/entry_64.S:905)


Thanks,
Sasha
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