Re: [PATCHv7 6/8] printk: use printk_safe buffers in printk

[Sergey Senozhatsky]

Hello Ross,

I was offline for a week for personal reasons.

On (01/31/17 10:27), Ross Zwisler wrote:
[..]
> [   13.090634] ======================================================
> [   13.090634] [ INFO: possible circular locking dependency detected ]
> [   13.090635] 4.10.0-rc5-mm1-00313-g5c0c3d7-dirty #10 Not tainted
> [   13.090635] -------------------------------------------------------
> [   13.090635] systemd/1 is trying to acquire lock:
> [   13.090636]  ((console_sem).lock){-.....}, at: [<ffffffff81110194>]
> down_trylock+0x14/0x40
> [   13.090637]
> [   13.090637] but task is already holding lock:
> [   13.090637]  (&rq->lock){-.-.-.}, at: [<ffffffff810e6116>]
> task_rq_lock+0x56/0xd0
> [   13.090638]
> [   13.090639] which lock already depends on the new lock.
> [   13.090639]
> [   13.090639]
> [   13.090640] the existing dependency chain (in reverse order) is:
> [   13.090640] c
> [   13.090640] -> #2 (&rq->lock){-.-.-.}:
> [   13.090641]        [<ffffffff8111727d>] lock_acquire+0xfd/0x200
> [   13.090642]        [<ffffffff81ca3171>] _raw_spin_lock+0x41/0x80
> [   13.090642]        [<ffffffff810fa38a>] task_fork_fair+0x3a/0x100
> [   13.090642]        [<ffffffff810ea80d>] sched_fork+0x10d/0x2c0
> [   13.090643]        [<ffffffff810b05df>] copy_process.part.30+0x69f/0x2190
> [   13.090643]        [<ffffffff810b22c6>] _do_fork+0xf6/0x700
> [   13.090643]        [<ffffffff810b28f9>] kernel_thread+0x29/0x30
> [   13.090644]        [<ffffffff81c91572>] rest_init+0x22/0x140
> [   13.090644]        [<ffffffff8280b051>] start_kernel+0x461/0x482
> [   13.090644]        [<ffffffff8280a2d6>] x86_64_start_reservations+0x2a/0x2c
> [   13.090645]        [<ffffffff8280a424>] x86_64_start_kernel+0x14c/0x16f
> [   13.090645]        [<ffffffff810001c4>] verify_cpu+0x0/0xfc
> [   13.090645]
> [   13.090645] -> #1 (&p->pi_lock){-.-.-.}:
> [   13.090647]        [<ffffffff8111727d>] lock_acquire+0xfd/0x200
> [   13.090647]        [<ffffffff81ca3f79>] _raw_spin_lock_irqsave+0x59/0x93
> [   13.090647]        [<ffffffff810e943f>] try_to_wake_up+0x3f/0x530
> [   13.090648]        [<ffffffff810e9945>] wake_up_process+0x15/0x20
> [   13.090648]        [<ffffffff81c9fc4c>] __up.isra.0+0x4c/0x50
> [   13.090648]        [<ffffffff81110256>] up+0x46/0x50
> [   13.090649]        [<ffffffff81128a55>] __up_console_sem+0x45/0x80
> [   13.090649]        [<ffffffff81129baf>] console_unlock+0x29f/0x5e0
> [   13.090649]        [<ffffffff8112a1c0>] vprintk_emit+0x2d0/0x3a0
> [   13.090650]        [<ffffffff8112a429>] vprintk_default+0x29/0x50
> [   13.090650]        [<ffffffff8112b445>] vprintk_func+0x25/0x80
> [   13.090650]        [<ffffffff81205c6d>] printk+0x52/0x6e
> [   13.090651]        [<ffffffff81185bdc>] kauditd_hold_skb+0x9c/0xa0
> [   13.090651]        [<ffffffff8118601b>] kauditd_thread+0x23b/0x520
> [   13.090651]        [<ffffffff810dbb6f>] kthread+0x10f/0x150
> [   13.090652]        [<ffffffff81ca42c1>] ret_from_fork+0x31/0x40
> [   13.090652]
> [   13.090652] -> #0 ((console_sem).lock){-.....}:
> [   13.090653]        [<ffffffff81116c85>] __lock_acquire+0x10e5/0x1270
> [   13.090653]        [<ffffffff8111727d>] lock_acquire+0xfd/0x200
> [   13.090654]        [<ffffffff81ca3f79>] _raw_spin_lock_irqsave+0x59/0x93
> [   13.090654]        [<ffffffff81110194>] down_trylock+0x14/0x40
> [   13.090654]        [<ffffffff81128b7c>] __down_trylock_console_sem+0x3c/0xc0
> [   13.090655]        [<ffffffff81128c16>] console_trylock+0x16/0x90
> [   13.090655]        [<ffffffff8112a1b7>] vprintk_emit+0x2c7/0x3a0
> [   13.090655]        [<ffffffff8112a429>] vprintk_default+0x29/0x50
> [   13.090656]        [<ffffffff8112b445>] vprintk_func+0x25/0x80
> [   13.090656]        [<ffffffff81205c6d>] printk+0x52/0x6e
> [   13.090656]        [<ffffffff810b3329>] __warn+0x39/0xf0
> [   13.090657]        [<ffffffff810b343f>] warn_slowpath_fmt+0x5f/0x80
> [   13.090657]        [<ffffffff810f550b>] update_load_avg+0x85b/0xb80
> [   13.090657]        [<ffffffff810f58bf>] detach_task_cfs_rq+0x3f/0x210
> [   13.090658]        [<ffffffff810f8e34>] task_change_group_fair+0x24/0x100
> [   13.090658]        [<ffffffff810e46ef>] sched_change_group+0x5f/0x110
> [   13.090658]        [<ffffffff810efd03>] sched_move_task+0x53/0x160
> [   13.090659]        [<ffffffff810efe46>] cpu_cgroup_attach+0x36/0x70
> [   13.090659]        [<ffffffff81172aa0>] cgroup_migrate_execute+0x230/0x3f0
> [   13.090659]        [<ffffffff81172d2e>] cgroup_migrate+0xce/0x140
> [   13.090660]        [<ffffffff8117301f>] cgroup_attach_task+0x27f/0x3e0
> [   13.090660]        [<ffffffff81175b7e>] __cgroup_procs_write+0x30e/0x510
> [   13.090661]        [<ffffffff81175d94>] cgroup_procs_write+0x14/0x20
> [   13.090661]        [<ffffffff811700e4>] cgroup_file_write+0x44/0x1e0
> [   13.090661]        [<ffffffff8135a92c>] kernfs_fop_write+0x13c/0x1c0
> [   13.090662]        [<ffffffff812b8e07>] __vfs_write+0x37/0x160
> [   13.090662]        [<ffffffff812ba8ab>] vfs_write+0xcb/0x1f0
> [   13.090662]        [<ffffffff812bbe38>] SyS_write+0x58/0xc0
> [   13.090663]        [<ffffffff81ca4041>] entry_SYSCALL_64_fastpath+0x1f/0xc2


... so it looks like a printk() from the scheduler; I don't think it's
related to printk_safe(). and to make it worse, printk is not in safe mode
when sched_move_task()->update_load_avg() calls it... so the system is
free to deadlock. we need printk_deferred() patch here.

I may be wrong, I just saw the message -- so this is a rather quick reply.


does the patch below fix the problem for you?
https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?h=sched/core&id=1b1d62254df0fe42a711eb71948f915918987790


	-ss