On Mon, 2019-09-23 at 14:58 +0200, Petr Mladek wrote:
> On Mon 2019-09-23 19:21:00, Sergey Senozhatsky wrote:
> > So we have
> >
> > port->lock -> MM -> zone->lock
> > // from pty_write()->__tty_buffer_request_room()->kmalloc()
> >
> > vs
> >
> > zone->lock -> printk() -> port->lock
> > // from __offline_pages()->__offline_isolated_pages()->printk()
>
> If I understand it correctly then this is the re-appearing problem.
> The only systematic solution with the current approach is to
> take port->lock in printk_safe/printk_deferred context.
>
> But this is a massive change that almost nobody wants. Instead,
> we want the changes that were discussed on Plumbers.
>
> Now, the question is what to do with existing kernels. There were
> several lockdep reports. And I am a bit lost. Did anyone seen
> real deadlocks or just the lockdep reports?
Yes, there is a real deadlock which causes an arm64 system hang on boot where
replace printk() with printk_deferred() in _warn_unseeded_randomness() will fix
it.
[ 1078.214683][T43784] WARNING: possible circular locking dependency detected
[ 1078.221550][T43784] 5.3.0-rc7-next-20190904 #14 Not tainted
[ 1078.227112][T43784] ------------------------------------------------------
[ 1078.233976][T43784] vi/43784 is trying to acquire lock:
[ 1078.239192][T43784] ffff008b7cff9290 (&(&zone->lock)->rlock){-.-.}, at:
rmqueue_bulk.constprop.21+0xb0/0x1218
[ 1078.249111][T43784]
[ 1078.249111][T43784] but task is already holding lock:
[ 1078.256322][T43784] ffff00938db47d40 (&(&port->lock)->rlock){-.-.}, at:
pty_write+0x78/0x100
[ 1078.264760][T43784]
[ 1078.264760][T43784] which lock already depends on the new lock.
[ 1078.264760][T43784]
[ 1078.275008][T43784]
[ 1078.275008][T43784] the existing dependency chain (in reverse order) is:
[ 1078.283869][T43784]
[ 1078.283869][T43784] -> #3 (&(&port->lock)->rlock){-.-.}:
[ 1078.291350][T43784] __lock_acquire+0x5c8/0xbb0
[ 1078.296394][T43784] lock_acquire+0x154/0x428
[ 1078.301266][T43784] _raw_spin_lock_irqsave+0x80/0xa0
[ 1078.306831][T43784] tty_port_tty_get+0x28/0x68
[ 1078.311873][T43784] tty_port_default_wakeup+0x20/0x40
[ 1078.317523][T43784] tty_port_tty_wakeup+0x38/0x48
[ 1078.322827][T43784] uart_write_wakeup+0x2c/0x50
[ 1078.327956][T43784] pl011_tx_chars+0x240/0x260
[ 1078.332999][T43784] pl011_start_tx+0x24/0xa8
[ 1078.337868][T43784] __uart_start+0x90/0xa0
[ 1078.342563][T43784] uart_write+0x15c/0x2c8
[ 1078.347261][T43784] do_output_char+0x1c8/0x2b0
[ 1078.352304][T43784] n_tty_write+0x300/0x668
[ 1078.357087][T43784] tty_write+0x2e8/0x430
[ 1078.361696][T43784] redirected_tty_write+0xcc/0xe8
[ 1078.367086][T43784] do_iter_write+0x228/0x270
[ 1078.372041][T43784] vfs_writev+0x10c/0x1c8
[ 1078.376735][T43784] do_writev+0xdc/0x180
[ 1078.381257][T43784] __arm64_sys_writev+0x50/0x60
[ 1078.386476][T43784] el0_svc_handler+0x11c/0x1f0
[ 1078.391606][T43784] el0_svc+0x8/0xc
[ 1078.395691][T43784]
[ 1078.395691][T43784] -> #2 (&port_lock_key){-.-.}:
[ 1078.402561][T43784] __lock_acquire+0x5c8/0xbb0
[ 1078.407604][T43784] lock_acquire+0x154/0x428
[ 1078.412474][T43784] _raw_spin_lock+0x68/0x88
[ 1078.417343][T43784] pl011_console_write+0x2ac/0x318
[ 1078.422820][T43784] console_unlock+0x3c4/0x898
[ 1078.427863][T43784] vprintk_emit+0x2d4/0x460
[ 1078.432732][T43784] vprintk_default+0x48/0x58
[ 1078.437688][T43784] vprintk_func+0x194/0x250
[ 1078.442557][T43784] printk+0xbc/0xec
[ 1078.446732][T43784] register_console+0x4a8/0x580
[ 1078.451947][T43784] uart_add_one_port+0x748/0x878
[ 1078.457250][T43784] pl011_register_port+0x98/0x128
[ 1078.462639][T43784] sbsa_uart_probe+0x398/0x480
[ 1078.467772][T43784] platform_drv_probe+0x70/0x108
[ 1078.473075][T43784] really_probe+0x15c/0x5d8
[ 1078.477944][T43784] driver_probe_device+0x94/0x1d0
[ 1078.483335][T43784] __device_attach_driver+0x11c/0x1a8
[ 1078.489072][T43784] bus_for_each_drv+0xf8/0x158
[ 1078.494201][T43784] __device_attach+0x164/0x240
[ 1078.499331][T43784] device_initial_probe+0x24/0x30
[ 1078.504721][T43784] bus_probe_device+0xf0/0x100
[ 1078.509850][T43784] device_add+0x63c/0x960
[ 1078.514546][T43784] platform_device_add+0x1ac/0x3b8
[ 1078.520023][T43784] platform_device_register_full+0x1fc/0x290
[ 1078.526373][T43784] acpi_create_platform_device.part.0+0x264/0x3a8
[ 1078.533152][T43784] acpi_create_platform_device+0x68/0x80
[ 1078.539150][T43784] acpi_default_enumeration+0x34/0x78
[ 1078.544887][T43784] acpi_bus_attach+0x340/0x3b8
[ 1078.550015][T43784] acpi_bus_attach+0xf8/0x3b8
[ 1078.555057][T43784] acpi_bus_attach+0xf8/0x3b8
[ 1078.560099][T43784] acpi_bus_attach+0xf8/0x3b8
[ 1078.565142][T43784] acpi_bus_scan+0x9c/0x100
[ 1078.570015][T43784] acpi_scan_init+0x16c/0x320
[ 1078.575058][T43784] acpi_init+0x330/0x3b8
[ 1078.579666][T43784] do_one_initcall+0x158/0x7ec
[ 1078.584797][T43784] kernel_init_freeable+0x9a8/0xa70
[ 1078.590360][T43784] kernel_init+0x18/0x138
[ 1078.595055][T43784] ret_from_fork+0x10/0x1c
[ 1078.599835][T43784]
[ 1078.599835][T43784] -> #1 (console_owner){-...}:
[ 1078.606618][T43784] __lock_acquire+0x5c8/0xbb0
[ 1078.611661][T43784] lock_acquire+0x154/0x428
[ 1078.616530][T43784] console_unlock+0x298/0x898
[ 1078.621573][T43784] vprintk_emit+0x2d4/0x460
[ 1078.626442][T43784] vprintk_default+0x48/0x58
[ 1078.631398][T43784] vprintk_func+0x194/0x250
[ 1078.636267][T43784] printk+0xbc/0xec
[ 1078.640443][T43784] _warn_unseeded_randomness+0xb4/0xd0
[ 1078.646267][T43784] get_random_u64+0x4c/0x100
[ 1078.651224][T43784] add_to_free_area_random+0x168/0x1a0
[ 1078.657047][T43784] free_one_page+0x3dc/0xd08
[ 1078.662003][T43784] __free_pages_ok+0x490/0xd00
[ 1078.667132][T43784] __free_pages+0xc4/0x118
[ 1078.671914][T43784] __free_pages_core+0x2e8/0x428
[ 1078.677219][T43784] memblock_free_pages+0xa4/0xec
[ 1078.682522][T43784] memblock_free_all+0x264/0x330
[ 1078.687825][T43784] mem_init+0x90/0x148
[ 1078.692259][T43784] start_kernel+0x368/0x684
[ 1078.697126][T43784]
[ 1078.697126][T43784] -> #0 (&(&zone->lock)->rlock){-.-.}:
[ 1078.704604][T43784] check_prev_add+0x120/0x1138
[ 1078.709733][T43784] validate_chain+0x888/0x1270
[ 1078.714863][T43784] __lock_acquire+0x5c8/0xbb0
[ 1078.719906][T43784] lock_acquire+0x154/0x428
[ 1078.724776][T43784] _raw_spin_lock+0x68/0x88
[ 1078.729645][T43784] rmqueue_bulk.constprop.21+0xb0/0x1218
[ 1078.735643][T43784] get_page_from_freelist+0x898/0x24a0
[ 1078.741467][T43784] __alloc_pages_nodemask+0x2a8/0x1d08
[ 1078.747291][T43784] alloc_pages_current+0xb4/0x150
[ 1078.752682][T43784] allocate_slab+0xab8/0x2350
[ 1078.757725][T43784] new_slab+0x98/0xc0
[ 1078.762073][T43784] ___slab_alloc+0x66c/0xa30
[ 1078.767029][T43784] __slab_alloc+0x68/0xc8
[ 1078.771725][T43784] __kmalloc+0x3d4/0x658
[ 1078.776333][T43784] __tty_buffer_request_room+0xd4/0x220
[ 1078.782244][T43784] tty_insert_flip_string_fixed_flag+0x6c/0x128
[ 1078.788849][T43784] pty_write+0x98/0x100
[ 1078.793370][T43784] n_tty_write+0x2a0/0x668
[ 1078.798152][T43784] tty_write+0x2e8/0x430
[ 1078.802760][T43784] __vfs_write+0x5c/0xb0
[ 1078.807368][T43784] vfs_write+0xf0/0x230
[ 1078.811890][T43784] ksys_write+0xd4/0x180
[ 1078.816498][T43784] __arm64_sys_write+0x4c/0x60
[ 1078.821627][T43784] el0_svc_handler+0x11c/0x1f0
[ 1078.826756][T43784] el0_svc+0x8/0xc
[ 1078.830842][T43784]
[ 1078.830842][T43784] other info that might help us debug this:
[ 1078.830842][T43784]
[ 1078.840918][T43784] Chain exists of:
[ 1078.840918][T43784] &(&zone->lock)->rlock --> &port_lock_key --> &(&port-
>lock)->rlock
[ 1078.840918][T43784]
[ 1078.854731][T43784] Possible unsafe locking scenario:
[ 1078.854731][T43784]
[ 1078.862029][T43784] CPU0 CPU1
[ 1078.867243][T43784] ---- ----
[ 1078.872457][T43784] lock(&(&port->lock)->rlock);
[ 1078.877238][T43784] lock(&port_lock_key);
[ 1078.883929][T43784] lock(&(&port->lock)-
>rlock);
[ 1078.891228][T43784] lock(&(&zone->lock)->rlock);
[ 1078.896010][T43784]
[ 1078.896010][T43784] *** DEADLOCK ***
[ 1078.896010][T43784]
[ 1078.904004][T43784] 5 locks held by vi/43784:
[ 1078.908351][T43784] #0: ffff000c36240890 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x44/0x50
[ 1078.917133][T43784] #1: ffff000c36240918 (&tty->atomic_write_lock){+.+.},
at: tty_write_lock+0x24/0x60
[ 1078.926521][T43784] #2: ffff000c36240aa0 (&o_tty->termios_rwsem/1){++++},
at: n_tty_write+0x108/0x668
[ 1078.935823][T43784] #3: ffffa0001e0b2360 (&ldata->output_lock){+.+.}, at:
n_tty_write+0x1d0/0x668
[ 1078.944777][T43784] #4: ffff00938db47d40 (&(&port->lock)->rlock){-.-.}, at:
pty_write+0x78/0x100
[ 1078.953644][T43784]
[ 1078.953644][T43784] stack backtrace:
[ 1078.959382][T43784] CPU: 97 PID: 43784 Comm: vi Not tainted 5.3.0-rc7-next-
20190904 #14
[ 1078.967376][T43784] Hardware name: HPE Apollo
70 /C01_APACHE_MB , BIOS L50_5.13_1.11 06/18/2019
[ 1078.977799][T43784] Call trace:
[ 1078.980932][T43784] dump_backtrace+0x0/0x228
[ 1078.985279][T43784] show_stack+0x24/0x30
[ 1078.989282][T43784] dump_stack+0xe8/0x13c
[ 1078.993370][T43784] print_circular_bug+0x334/0x3d8
[ 1078.998240][T43784] check_noncircular+0x268/0x310
[ 1079.003022][T43784] check_prev_add+0x120/0x1138
[ 1079.007631][T43784] validate_chain+0x888/0x1270
[ 1079.012241][T43784] __lock_acquire+0x5c8/0xbb0
[ 1079.016763][T43784] lock_acquire+0x154/0x428
[ 1079.021111][T43784] _raw_spin_lock+0x68/0x88
[ 1079.025460][T43784] rmqueue_bulk.constprop.21+0xb0/0x1218
[ 1079.030937][T43784] get_page_from_freelist+0x898/0x24a0
[ 1079.036240][T43784] __alloc_pages_nodemask+0x2a8/0x1d08
[ 1079.041542][T43784] alloc_pages_current+0xb4/0x150
[ 1079.046412][T43784] allocate_slab+0xab8/0x2350
[ 1079.050934][T43784] new_slab+0x98/0xc0
[ 1079.054761][T43784] ___slab_alloc+0x66c/0xa30
[ 1079.059196][T43784] __slab_alloc+0x68/0xc8
[ 1079.063371][T43784] __kmalloc+0x3d4/0x658
[ 1079.067458][T43784] __tty_buffer_request_room+0xd4/0x220
[ 1079.072847][T43784] tty_insert_flip_string_fixed_flag+0x6c/0x128
[ 1079.078932][T43784] pty_write+0x98/0x100
[ 1079.082932][T43784] n_tty_write+0x2a0/0x668
[ 1079.087193][T43784] tty_write+0x2e8/0x430
[ 1079.091280][T43784] __vfs_write+0x5c/0xb0
[ 1079.095367][T43784] vfs_write+0xf0/0x230
[ 1079.099368][T43784] ksys_write+0xd4/0x180
[ 1079.103455][T43784] __arm64_sys_write+0x4c/0x60
[ 1079.108064][T43784] el0_svc_handler+0x11c/0x1f0
[ 1079.112672][T43784] el0_svc+0x8/0xc
>
> To be clear. I would feel more comfortable when there are no
> deadlocks. But I also do not want to invest too much time
> into old kernels. All these problems were there for ages.
> We could finally see them because lockdep was enabled in printk()
> thanks to printk_safe. Well, it is getting worse over time with
> the increasing complexity and number of debugging messages.
>
> > A number of debugging options make the kernel less stable.
> > Sad but true.
>
> Yeah. The only good thing is that most debug options are not
> enabled on production systems. It is not an excuse for ignoring
> the problems. But it might be important for prioritizing.
If you look at the above splat and the one I just sent for offline_pages()-
>dump_page() splat. They don't involve any debug options, so they could happen
on production systems if the timing is right (which could be tricky).
