Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier()dependency on __stop_machine()")

From: Srivatsa S. Bhat
Date: Tue Oct 02 2012 - 16:40:03 EST

On 10/02/2012 09:44 PM, Jiri Kosina wrote:
> Hi,
>
> this commit:
>
> ==
> 1331e7a1bbe1f11b19c4327ba0853bee2a606543 is the first bad commit
> commit 1331e7a1bbe1f11b19c4327ba0853bee2a606543
> Author: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> Date: Thu Aug 2 17:43:50 2012 -0700
>
> rcu: Remove _rcu_barrier() dependency on __stop_machine()
>
> Currently, _rcu_barrier() relies on preempt_disable() to prevent
> any CPU from going offline, which in turn depends on CPU hotplug's
> use of __stop_machine().
>
> This patch therefore makes _rcu_barrier() use get_online_cpus() to
> block CPU-hotplug operations. This has the added benefit of removing
> the need for _rcu_barrier() to adopt callbacks: Because CPU-hotplug
> operations are excluded, there can be no callbacks to adopt. This
> commit simplifies the code accordingly.
>
> Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> ==
>
> is causing lockdep to complain (see the full trace below). I haven't yet
> had time to analyze what exactly is happening, and probably will not have
> time to do so until tomorrow, so just sending this as a heads-up in case
> anyone sees the culprit immediately.
>
> ======================================================
> [ INFO: possible circular locking dependency detected ]
> 3.6.0-rc5-00004-g0d8ee37 #143 Not tainted
> -------------------------------------------------------
> kworker/u:2/40 is trying to acquire lock:
> (rcu_sched_state.barrier_mutex){+.+...}, at: [<ffffffff810f2126>] _rcu_barrier+0x26/0x1e0
>
> but task is already holding lock:
> (slab_mutex){+.+.+.}, at: [<ffffffff81176e15>] kmem_cache_destroy+0x45/0xe0
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #2 (slab_mutex){+.+.+.}:
> [<ffffffff810ae1e2>] validate_chain+0x632/0x720
> [<ffffffff810ae5d9>] __lock_acquire+0x309/0x530
> [<ffffffff810ae921>] lock_acquire+0x121/0x190
> [<ffffffff8155d4cc>] __mutex_lock_common+0x5c/0x450
> [<ffffffff8155d9ee>] mutex_lock_nested+0x3e/0x50
> [<ffffffff81558cb5>] cpuup_callback+0x2f/0xbe
> [<ffffffff81564b83>] notifier_call_chain+0x93/0x140
> [<ffffffff81076f89>] __raw_notifier_call_chain+0x9/0x10
> [<ffffffff8155719d>] _cpu_up+0xba/0x14e
> [<ffffffff815572ed>] cpu_up+0xbc/0x117
> [<ffffffff81ae05e3>] smp_init+0x6b/0x9f
> [<ffffffff81ac47d6>] kernel_init+0x147/0x1dc
> [<ffffffff8156ab44>] kernel_thread_helper+0x4/0x10
>
> -> #1 (cpu_hotplug.lock){+.+.+.}:
> [<ffffffff810ae1e2>] validate_chain+0x632/0x720
> [<ffffffff810ae5d9>] __lock_acquire+0x309/0x530
> [<ffffffff810ae921>] lock_acquire+0x121/0x190
> [<ffffffff8155d4cc>] __mutex_lock_common+0x5c/0x450
> [<ffffffff8155d9ee>] mutex_lock_nested+0x3e/0x50
> [<ffffffff81049197>] get_online_cpus+0x37/0x50
> [<ffffffff810f21bb>] _rcu_barrier+0xbb/0x1e0
> [<ffffffff810f22f0>] rcu_barrier_sched+0x10/0x20
> [<ffffffff810f2309>] rcu_barrier+0x9/0x10
> [<ffffffff8118c129>] deactivate_locked_super+0x49/0x90
> [<ffffffff8118cc01>] deactivate_super+0x61/0x70
> [<ffffffff811aaaa7>] mntput_no_expire+0x127/0x180
> [<ffffffff811ab49e>] sys_umount+0x6e/0xd0
> [<ffffffff81569979>] system_call_fastpath+0x16/0x1b
>
> -> #0 (rcu_sched_state.barrier_mutex){+.+...}:
> [<ffffffff810adb4e>] check_prev_add+0x3de/0x440
> [<ffffffff810ae1e2>] validate_chain+0x632/0x720
> [<ffffffff810ae5d9>] __lock_acquire+0x309/0x530
> [<ffffffff810ae921>] lock_acquire+0x121/0x190
> [<ffffffff8155d4cc>] __mutex_lock_common+0x5c/0x450
> [<ffffffff8155d9ee>] mutex_lock_nested+0x3e/0x50
> [<ffffffff810f2126>] _rcu_barrier+0x26/0x1e0
> [<ffffffff810f22f0>] rcu_barrier_sched+0x10/0x20
> [<ffffffff810f2309>] rcu_barrier+0x9/0x10
> [<ffffffff81176ea1>] kmem_cache_destroy+0xd1/0xe0
> [<ffffffffa04c3154>] nf_conntrack_cleanup_net+0xe4/0x110 [nf_conntrack]
> [<ffffffffa04c31aa>] nf_conntrack_cleanup+0x2a/0x70 [nf_conntrack]
> [<ffffffffa04c42ce>] nf_conntrack_net_exit+0x5e/0x80 [nf_conntrack]
> [<ffffffff81454b79>] ops_exit_list+0x39/0x60
> [<ffffffff814551ab>] cleanup_net+0xfb/0x1b0
> [<ffffffff8106917b>] process_one_work+0x26b/0x4c0
> [<ffffffff81069f3e>] worker_thread+0x12e/0x320
> [<ffffffff8106f73e>] kthread+0x9e/0xb0
> [<ffffffff8156ab44>] kernel_thread_helper+0x4/0x10
>
> other info that might help us debug this:
>
> Chain exists of:
> rcu_sched_state.barrier_mutex --> cpu_hotplug.lock --> slab_mutex
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(slab_mutex);
> lock(cpu_hotplug.lock);
> lock(slab_mutex);
> lock(rcu_sched_state.barrier_mutex);
>
> *** DEADLOCK ***
>
> 4 locks held by kworker/u:2/40:
> #0: (netns){.+.+.+}, at: [<ffffffff810690b2>] process_one_work+0x1a2/0x4c0
> #1: (net_cleanup_work){+.+.+.}, at: [<ffffffff810690b2>] process_one_work+0x1a2/0x4c0
> #2: (net_mutex){+.+.+.}, at: [<ffffffff81455130>] cleanup_net+0x80/0x1b0
> #3: (slab_mutex){+.+.+.}, at: [<ffffffff81176e15>] kmem_cache_destroy+0x45/0xe0
>

I don't see how this circular locking dependency can occur.. If you are using SLUB,
kmem_cache_destroy() releases slab_mutex before it calls rcu_barrier(). If you are
using SLAB, kmem_cache_destroy() wraps its whole operation inside get/put_online_cpus(),
which means, it cannot run concurrently with a hotplug operation such as cpu_up(). So, I'm
rather puzzled at this lockdep splat..

Regards,
Srivatsa S. Bhat

> stack backtrace:
> Pid: 40, comm: kworker/u:2 Not tainted 3.6.0-rc5-00004-g0d8ee37 #143
> Call Trace:
> [<ffffffff810ac85f>] print_circular_bug+0x10f/0x120
> [<ffffffff810adb4e>] check_prev_add+0x3de/0x440
> [<ffffffff810ad85a>] ? check_prev_add+0xea/0x440
> [<ffffffff8102c72f>] ? flat_send_IPI_mask+0x7f/0xc0
> [<ffffffff810ae1e2>] validate_chain+0x632/0x720
> [<ffffffff810ae5d9>] __lock_acquire+0x309/0x530
> [<ffffffff810ae921>] lock_acquire+0x121/0x190
> [<ffffffff810f2126>] ? _rcu_barrier+0x26/0x1e0
> [<ffffffff8155d4cc>] __mutex_lock_common+0x5c/0x450
> [<ffffffff810f2126>] ? _rcu_barrier+0x26/0x1e0
> [<ffffffff810b5e45>] ? on_each_cpu+0x65/0xc0
> [<ffffffff810f2126>] ? _rcu_barrier+0x26/0x1e0
> [<ffffffff8155d9ee>] mutex_lock_nested+0x3e/0x50
> [<ffffffff810f2126>] _rcu_barrier+0x26/0x1e0
> [<ffffffff810f22f0>] rcu_barrier_sched+0x10/0x20
> [<ffffffff810f2309>] rcu_barrier+0x9/0x10
> [<ffffffff81176ea1>] kmem_cache_destroy+0xd1/0xe0
> [<ffffffffa04c3154>] nf_conntrack_cleanup_net+0xe4/0x110 [nf_conntrack]
> [<ffffffffa04c31aa>] nf_conntrack_cleanup+0x2a/0x70 [nf_conntrack]
> [<ffffffffa04c42ce>] nf_conntrack_net_exit+0x5e/0x80 [nf_conntrack]
> [<ffffffff81454b79>] ops_exit_list+0x39/0x60
> [<ffffffff814551ab>] cleanup_net+0xfb/0x1b0
> [<ffffffff8106917b>] process_one_work+0x26b/0x4c0
> [<ffffffff810690b2>] ? process_one_work+0x1a2/0x4c0
> [<ffffffff81069e69>] ? worker_thread+0x59/0x320
> [<ffffffff814550b0>] ? net_drop_ns+0x40/0x40
> [<ffffffff81069f3e>] worker_thread+0x12e/0x320
> [<ffffffff81069e10>] ? manage_workers+0x110/0x110
> [<ffffffff8106f73e>] kthread+0x9e/0xb0
> [<ffffffff8156ab44>] kernel_thread_helper+0x4/0x10
> [<ffffffff81560b70>] ? retint_restore_args+0x13/0x13
> [<ffffffff8106f6a0>] ? __init_kthread_worker+0x70/0x70
> [<ffffffff8156ab40>] ? gs_change+0x13/0x13
>
>

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