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Date: Wed, 7 Apr 2021 11:20:58 +0200
From: Petr Mladek <pmladek@...e.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
Jiri Slaby <jirislaby@...nel.org>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Steven Rostedt <rostedt@...dmis.org>,
John Ogness <john.ogness@...utronix.de>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: use printk_safe context at tty_msg()
On Tue 2021-04-06 21:10:48, Greg Kroah-Hartman wrote:
> On Wed, Apr 07, 2021 at 01:22:34AM +0900, Tetsuo Handa wrote:
> > On 2021/04/07 0:10, Petr Mladek wrote:
> > >> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
> > >> index 6d4995a5f318..d59f7873bc49 100644
> > >> --- a/drivers/tty/tty_buffer.c
> > >> +++ b/drivers/tty/tty_buffer.c
> > >> @@ -156,6 +156,7 @@ static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
> > >> {
> > >> struct llist_node *free;
> > >> struct tty_buffer *p;
> > >> + unsigned long flags;
> > >>
> > >> /* Round the buffer size out */
> > >> size = __ALIGN_MASK(size, TTYB_ALIGN_MASK);
> > >> @@ -172,7 +173,9 @@ static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
> > >> have queued and recycle that ? */
> > >> if (atomic_read(&port->buf.mem_used) > port->buf.mem_limit)
> > >> return NULL;
> > >> - p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
> > >> + printk_safe_enter_irqsave(flags);
> > >> + p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC | __GFP_NOWARN);
> > >> + printk_safe_exit_irqrestore(flags);
> > >
> > > I do not see tty_buffer_alloc() anywhere at
> > > https://syzkaller.appspot.com/bug?id=39ea6caa479af471183997376dc7e90bc7d64a6a
> > >
> > > Could you please provide more details why this is needed?
> >
> > Quoting from https://syzkaller.appspot.com/text?tag=CrashLog&x=131d4ddf800000 at https://syzkaller.appspot.com/bug?id=bda1a87bea05a9072003e6447c44b03ca1492b1c :
> >
> > [ 274.992629] FAULT_INJECTION: forcing a failure.
> > [ 274.992629] name failslab, interval 1, probability 0, space 0, times 0
> > [ 274.994085] CPU: 1 PID: 19187 Comm: syz-executor6 Not tainted 4.17.0+ #89
> > [ 274.994094] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > [ 274.994099] Call Trace:
> > [ 274.994120] dump_stack+0x1b9/0x294
> > [ 274.994145] ? dump_stack_print_info.cold.2+0x52/0x52
> > [ 275.270563] should_fail.cold.4+0xa/0x1a
> > [ 275.274639] ? fault_create_debugfs_attr+0x1f0/0x1f0
> > [ 275.279747] ? graph_lock+0x170/0x170
> > [ 275.283541] ? check_noncircular+0x20/0x20
> > [ 275.287788] ? debug_check_no_locks_freed+0x310/0x310
> > [ 275.292983] ? lock_acquire+0x1dc/0x520
> > [ 275.296961] ? process_echoes+0xb6/0x170
> > [ 275.301021] ? check_same_owner+0x320/0x320
> > [ 275.305347] ? print_usage_bug+0xc0/0xc0
> > [ 275.309404] ? __might_sleep+0x95/0x190
> > [ 275.313394] ? __sanitizer_cov_trace_const_cmp8+0x18/0x20
> > [ 275.318934] __should_failslab+0x124/0x180
> > [ 275.323168] should_failslab+0x9/0x14
> > [ 275.326974] __kmalloc+0x63/0x760
> > [ 275.330423] ? mutex_trylock+0x2a0/0x2a0
> > [ 275.334479] ? print_usage_bug+0xc0/0xc0
> > [ 275.338539] ? __tty_buffer_request_room+0x2d2/0x7f0
> > [ 275.343648] __tty_buffer_request_room+0x2d2/0x7f0
> > [ 275.348576] ? flush_to_ldisc+0x560/0x560
> > [ 275.352717] ? lock_acquire+0x1dc/0x520
> > [ 275.356698] ? pty_write+0xf9/0x1f0
> > [ 275.360325] ? lock_release+0xa10/0xa10
> > [ 275.364293] ? kasan_check_read+0x11/0x20
> > [ 275.368434] ? do_raw_spin_unlock+0x9e/0x2e0
> > [ 275.372831] ? do_raw_spin_trylock+0x1b0/0x1b0
> > [ 275.377411] tty_insert_flip_string_fixed_flag+0x8d/0x1f0
> > [ 275.382942] ? do_raw_spin_lock+0xc1/0x200
> > [ 275.387174] pty_write+0x12c/0x1f0
> > [ 275.390720] tty_put_char+0x129/0x150
> > [ 275.394525] ? dev_match_devt+0x90/0x90
> > [ 275.398498] ? pty_write_room+0xc9/0xf0
> > [ 275.402485] ? __sanitizer_cov_trace_switch+0x53/0x90
> > [ 275.407684] __process_echoes+0x4d9/0x8d0
> > [ 275.411829] ? _raw_spin_unlock_irqrestore+0x74/0xc0
> > [ 275.416936] process_echoes+0xfc/0x170
> > [ 275.420816] n_tty_set_termios+0xb56/0xe80
> > [ 275.425047] ? n_tty_receive_signal_char+0x120/0x120
> > [ 275.430148] tty_set_termios+0x7a0/0xac0
> > [ 275.434201] ? tty_wait_until_sent+0x5b0/0x5b0
> > [ 275.438782] ? __sanitizer_cov_trace_const_cmp8+0x18/0x20
> > [ 275.444318] set_termios+0x41e/0x7d0
> > [ 275.448026] ? tty_perform_flush+0x80/0x80
> > [ 275.452256] ? __sanitizer_cov_trace_const_cmp8+0x18/0x20
> > [ 275.457845] tty_mode_ioctl+0x535/0xb50
> > [ 275.461821] ? set_termios+0x7d0/0x7d0
> > [ 275.465740] ? check_same_owner+0x320/0x320
> > [ 275.470059] ? graph_lock+0x170/0x170
> > [ 275.473857] n_tty_ioctl_helper+0x54/0x3b0
> > [ 275.478085] n_tty_ioctl+0x54/0x320
> > [ 275.481711] ? ldsem_down_read+0x37/0x40
> > [ 275.485760] ? ldsem_down_read+0x37/0x40
> > [ 275.489836] tty_ioctl+0x5e1/0x1870
> > [ 275.493454] ? commit_echoes+0x1d0/0x1d0
> > [ 275.497506] ? tty_vhangup+0x30/0x30
> > [ 275.501212] ? rcu_is_watching+0x85/0x140
> > [ 275.505353] ? rcu_report_qs_rnp+0x790/0x790
> > [ 275.509757] ? __fget+0x40c/0x650
> > [ 275.513243] ? find_held_lock+0x31/0x1c0
> > [ 275.517439] ? expand_files.part.8+0x9a0/0x9a0
> > [ 275.522033] ? kasan_check_write+0x14/0x20
> > [ 275.526389] ? __mutex_unlock_slowpath+0x180/0x8a0
> > [ 275.531314] ? wait_for_completion+0x870/0x870
> > [ 275.535907] ? tty_vhangup+0x30/0x30
> > [ 275.539616] do_vfs_ioctl+0x1cf/0x16f0
> > [ 275.543522] ? ioctl_preallocate+0x2e0/0x2e0
> > [ 275.547934] ? fget_raw+0x20/0x20
> > [ 275.551554] ? __sb_end_write+0xac/0xe0
> > [ 275.555525] ? __sanitizer_cov_trace_const_cmp1+0x1a/0x20
> > [ 275.561064] ? fput+0x130/0x1a0
> > [ 275.564345] ? ksys_write+0x1a6/0x250
> > [ 275.568144] ? security_file_ioctl+0x94/0xc0
> > [ 275.572576] ksys_ioctl+0xa9/0xd0
> > [ 275.576028] __x64_sys_ioctl+0x73/0xb0
> > [ 275.579911] do_syscall_64+0x1b1/0x800
> > [ 275.583789] ? finish_task_switch+0x1ca/0x840
> > [ 275.588301] ? syscall_return_slowpath+0x5c0/0x5c0
> > [ 275.593310] ? syscall_return_slowpath+0x30f/0x5c0
> > [ 275.598239] ? entry_SYSCALL_64_after_hwframe+0x59/0xbe
> > [ 275.603605] ? trace_hardirqs_off_thunk+0x1a/0x1c
> > [ 275.608463] entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > [ 275.613649] RIP: 0033:0x4559f9
> > [ 275.616823] Code: 1d ba fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 eb b9 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> > [ 275.636056] RSP: 002b:00007f7ad0e91c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> > [ 275.643775] RAX: ffffffffffffffda RBX: 00007f7ad0e926d4 RCX: 00000000004559f9
> > [ 275.651034] RDX: 0000000020002000 RSI: 0000000000005403 RDI: 0000000000000015
> > [ 275.658293] RBP: 000000000072bea0 R08: 0000000000000000 R09: 0000000000000000
> > [ 275.665554] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000016
> > [ 275.672819] R13: 00000000004be500 R14: 00000000004ccad8 R15: 0000000000000000
> > [ 275.680137] CPU: 0 PID: 19202 Comm: syz-executor2 Not tainted 4.17.0+ #89
> > [ 275.680139]
> > [ 275.680149] ======================================================
> > [ 275.680154] WARNING: possible circular locking dependency detected
> > [ 275.680156] 4.17.0+ #89 Not tainted
> > [ 275.680160] ------------------------------------------------------
> > [ 275.680163] syz-executor6/19187 is trying to acquire lock:
> > [ 275.680165] 000000006e810651 (console_owner){-.-.}, at: vprintk_emit+0x826/0xde0
> > [ 275.680175]
> > [ 275.680178] but task is already holding lock:
> > [ 275.680181] 000000004ec3be2c (&(&port->lock)->rlock){-.-.}, at: pty_write+0xf9/0x1f0
> > [ 275.680192]
> > [ 275.680196] which lock already depends on the new lock.
> > [ 275.680197]
> > [ 275.680199]
> > [ 275.680203] the existing dependency chain (in reverse order) is:
> > [ 275.680205]
> > [ 275.680207] -> #2 (&(&port->lock)->rlock){-.-.}:
> > [ 275.680219] _raw_spin_lock_irqsave+0x96/0xc0
> > [ 275.680222] tty_port_tty_get+0x20/0x80
> > [ 275.680226] tty_port_default_wakeup+0x15/0x40
> > [ 275.680230] tty_port_tty_wakeup+0x5d/0x70
> > [ 275.680233] uart_write_wakeup+0x44/0x60
> > [ 275.680237] serial8250_tx_chars+0x4be/0xb60
> > [ 275.680241] serial8250_handle_irq.part.25+0x1ee/0x280
> > [ 275.680245] serial8250_default_handle_irq+0xc8/0x150
> > [ 275.680248] serial8250_interrupt+0xfa/0x1d0
> > [ 275.680252] __handle_irq_event_percpu+0x1c0/0xad0
> > [ 275.680256] handle_irq_event_percpu+0x98/0x1c0
> > [ 275.680259] handle_irq_event+0xa7/0x135
> > [ 275.680262] handle_edge_irq+0x20f/0x870
> > [ 275.680265] handle_irq+0x18c/0x2e7
> > [ 275.680268] do_IRQ+0x78/0x190
> > [ 275.680271] ret_from_intr+0x0/0x1e
> > [ 275.680274] native_safe_halt+0x6/0x10
> > [ 275.680277] default_idle+0xc2/0x440
> > [ 275.680280] arch_cpu_idle+0x10/0x20
> > [ 275.680283] default_idle_call+0x6d/0x90
> > [ 275.680286] do_idle+0x395/0x560
> > [ 275.680289] cpu_startup_entry+0x104/0x120
> > [ 275.680293] start_secondary+0x42b/0x5c0
> > [ 275.680296] secondary_startup_64+0xa5/0xb0
> > [ 275.680298]
> > [ 275.680300] -> #1 (&port_lock_key){-.-.}:
> > [ 275.680312] _raw_spin_lock_irqsave+0x96/0xc0
> > [ 275.680315] serial8250_console_write+0x8d5/0xb00
> > [ 275.680319] univ8250_console_write+0x5f/0x70
> > [ 275.680322] console_unlock+0xa9d/0x10a0
> > [ 275.680325] vprintk_emit+0x6b2/0xde0
> > [ 275.680329] vprintk_default+0x28/0x30
> > [ 275.680332] vprintk_func+0x7a/0xe7
> > [ 275.680335] printk+0x9e/0xba
> > [ 275.680338] register_console+0x7e7/0xc00
> > [ 275.680341] univ8250_console_init+0x3f/0x4b
> > [ 275.680344] console_init+0x6d9/0xa38
> > [ 275.680348] start_kernel+0x608/0x92d
> > [ 275.680351] x86_64_start_reservations+0x29/0x2b
> > [ 275.680354] x86_64_start_kernel+0x76/0x79
> > [ 275.680358] secondary_startup_64+0xa5/0xb0
> > [ 275.680359]
> > [ 275.680361] -> #0 (console_owner){-.-.}:
> > [ 275.680372] lock_acquire+0x1dc/0x520
> > [ 275.680375] vprintk_emit+0x884/0xde0
> > [ 275.680379] vprintk_default+0x28/0x30
> > [ 275.680382] vprintk_func+0x7a/0xe7
> > [ 275.680384] printk+0x9e/0xba
> > [ 275.680388] should_fail+0x97a/0xbcd
> > [ 275.680391] __should_failslab+0x124/0x180
> > [ 275.680394] should_failslab+0x9/0x14
> > [ 275.680397] __kmalloc+0x63/0x760
> > [ 275.680401] __tty_buffer_request_room+0x2d2/0x7f0
> > [ 275.680405] tty_insert_flip_string_fixed_flag+0x8d/0x1f0
> > [ 275.680408] pty_write+0x12c/0x1f0
> > [ 275.680411] tty_put_char+0x129/0x150
> > [ 275.680414] __process_echoes+0x4d9/0x8d0
> > [ 275.680418] process_echoes+0xfc/0x170
> > [ 275.680421] n_tty_set_termios+0xb56/0xe80
> > [ 275.680424] tty_set_termios+0x7a0/0xac0
> > [ 275.680427] set_termios+0x41e/0x7d0
> > [ 275.680430] tty_mode_ioctl+0x535/0xb50
> > [ 275.680434] n_tty_ioctl_helper+0x54/0x3b0
> > [ 275.680437] n_tty_ioctl+0x54/0x320
> > [ 275.680440] tty_ioctl+0x5e1/0x1870
> > [ 275.680443] do_vfs_ioctl+0x1cf/0x16f0
> > [ 275.680446] ksys_ioctl+0xa9/0xd0
> > [ 275.680449] __x64_sys_ioctl+0x73/0xb0
> > [ 275.680452] do_syscall_64+0x1b1/0x800
> > [ 275.680456] entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > [ 275.680458]
> > [ 275.680461] other info that might help us debug this:
> > [ 275.680463]
> > [ 275.680466] Chain exists of:
> > [ 275.680467] console_owner --> &port_lock_key --> &(&port->lock)->rlock
> > [ 275.680482]
> > [ 275.680485] Possible unsafe locking scenario:
> > [ 275.680487]
> > [ 275.680490] CPU0 CPU1
> > [ 275.680493] ---- ----
> > [ 275.680495] lock(&(&port->lock)->rlock);
> > [ 275.680503] lock(&port_lock_key);
> > [ 275.680511] lock(&(&port->lock)->rlock);
> > [ 275.680517] lock(console_owner);
> > [ 275.680524]
> > [ 275.680526] *** DEADLOCK ***
> > [ 275.680528]
> > [ 275.680531] 5 locks held by syz-executor6/19187:
> > [ 275.680533] #0: 000000007080a3ba (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x37/0x40
> > [ 275.680547] #1: 000000006fe00659 (&o_tty->termios_rwsem/1){++++}, at: tty_set_termios+0xfd/0xac0
> > [ 275.680563] #2: 000000007080a3ba (&tty->ldisc_sem){++++}, at: tty_ldisc_ref+0x22/0x90
> > [ 275.680576] #3: 00000000f8d1ab16 (&ldata->output_lock){+.+.}, at: process_echoes+0xb6/0x170
> > [ 275.680589] #4: 000000004ec3be2c (&(&port->lock)->rlock){-.-.}, at: pty_write+0xf9/0x1f0
> > [ 275.680603]
> > [ 275.680605] stack backtrace:
> > [ 275.680610] CPU: 1 PID: 19187 Comm: syz-executor6 Not tainted 4.17.0+ #89
> > [ 275.680616] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > [ 275.680618] Call Trace:
> > [ 275.680621] dump_stack+0x1b9/0x294
> > [ 275.680625] ? dump_stack_print_info.cold.2+0x52/0x52
> > [ 275.680628] ? vprintk_func+0xd0/0xe7
> > [ 275.680632] print_circular_bug.isra.36.cold.56+0x1bd/0x27d
> > [ 275.680636] ? save_trace+0xe0/0x290
> > [ 275.680639] __lock_acquire+0x343e/0x5140
> > [ 275.680643] ? debug_check_no_locks_freed+0x310/0x310
> > [ 275.680646] ? check_noncircular+0x20/0x20
> > [ 275.680649] ? dentry_name+0x687/0x930
> > [ 275.680653] ? restricted_pointer+0x400/0x400
> > [ 275.680656] ? __lock_acquire+0x7f5/0x5140
> > [ 275.680659] ? print_usage_bug+0xc0/0xc0
> > [ 275.680662] ? graph_lock+0x170/0x170
> > [ 275.680665] ? graph_lock+0x170/0x170
> > [ 275.680668] ? graph_lock+0x170/0x170
> > [ 275.680672] ? find_held_lock+0x36/0x1c0
> > [ 275.680675] lock_acquire+0x1dc/0x520
> > [ 275.680678] ? vprintk_emit+0x826/0xde0
> > [ 275.680681] ? lock_release+0xa10/0xa10
> > [ 275.680684] ? kasan_check_read+0x11/0x20
> > [ 275.680687] ? do_raw_spin_unlock+0x9e/0x2e0
> > [ 275.680691] ? do_raw_spin_trylock+0x1b0/0x1b0
> > [ 275.680694] ? kasan_check_write+0x14/0x20
> > [ 275.680698] ? do_raw_spin_lock+0xc1/0x200
> > [ 275.680701] vprintk_emit+0x884/0xde0
> > [ 275.680704] ? vprintk_emit+0x826/0xde0
> > [ 275.680707] ? wake_up_klogd+0x100/0x100
> > [ 275.680711] ? debug_check_no_locks_freed+0x310/0x310
> > [ 275.680714] ? print_usage_bug+0xc0/0xc0
> > [ 275.680717] ? print_usage_bug+0xc0/0xc0
> > [ 275.680720] vprintk_default+0x28/0x30
> > [ 275.680723] vprintk_func+0x7a/0xe7
> > [ 275.680726] printk+0x9e/0xba
> > [ 275.680730] ? kmsg_dump_rewind_nolock+0xe4/0xe4
> > [ 275.680733] should_fail+0x97a/0xbcd
> > [ 275.680737] ? fault_create_debugfs_attr+0x1f0/0x1f0
> > [ 275.680740] ? graph_lock+0x170/0x170
> > [ 275.680743] ? check_noncircular+0x20/0x20
> > [ 275.680747] ? debug_check_no_locks_freed+0x310/0x310
> > [ 275.680750] ? lock_acquire+0x1dc/0x520
> > [ 275.680753] ? process_echoes+0xb6/0x170
> > [ 275.680757] ? check_same_owner+0x320/0x320
> > [ 275.680760] ? print_usage_bug+0xc0/0xc0
> > [ 275.680763] ? __might_sleep+0x95/0x190
> > [ 275.680767] ? __sanitizer_cov_trace_const_cmp8+0x18/0x20
> > [ 275.680770] __should_failslab+0x124/0x180
> > [ 275.680774] should_failslab+0x9/0x14
> > [ 275.680776] __kmalloc+0x63/0x760
> > [ 275.680780] ? mutex_trylock+0x2a0/0x2a0
> > [ 275.680783] ? print_usage_bug+0xc0/0xc0
> > [ 275.680787] ? __tty_buffer_request_room+0x2d2/0x7f0
> > [ 275.680790] __tty_buffer_request_room+0x2d2/0x7f0
> > [ 275.680794] ? flush_to_ldisc+0x560/0x560
> > [ 275.680797] ? lock_acquire+0x1dc/0x520
> > [ 275.680800] ? pty_write+0xf9/0x1f0
> > [ 275.680803] ? lock_release+0xa10/0xa10
> > [ 275.680806] ? kasan_check_read+0x11/0x20
> > [ 275.680810] ? do_raw_spin_unlock+0x9e/0x2e0
> > [ 275.680813] ? do_raw_spin_trylock+0x1b0/0x1b0
> > [ 275.680817] tty_insert_flip_string_fixed_flag+0x8d/0x1f0
> > [ 275.680821] ? do_raw_spin_lock+0xc1/0x200
> > [ 275.680823] pty_write+0x12c/0x1f0
> > [ 275.680827] tty_put_char+0x129/0x150
> > [ 275.680830] ? dev_match_devt+0x90/0x90
> > [ 275.680833] ? pty_write_room+0xc9/0xf0
> > [ 275.680837] ? __sanitizer_cov_trace_switch+0x53/0x90
> > [ 275.680840] __process_echoes+0x4d9/0x8d0
> > [ 275.680844] ? _raw_spin_unlock_irqrestore+0x74/0xc0
> > [ 275.680847] process_echoes+0xfc/0x170
> > [ 275.680850] n_tty_set_termios+0xb56/0xe80
> > [ 275.680854] ? n_tty_receive_signal_char+0x120/0x120
> > [ 275.680857] tty_set_termios+0x7a0/0xac0
> > [ 275.680861] ? tty_wait_until_sent+0x5b0/0x5b0
> > [ 275.680865] ? __sanitizer_cov_trace_const_cmp8+0x18/0x20
> > [ 275.680868] set_termios+0x41e/0x7d0
> > [ 275.680871] ? tty_perform_flush+0x80/0x80
> > [ 275.680875] ? __sanitizer_cov_trace_const_cmp8+0x18/0x20
> > [ 275.680878] tty_mode_ioctl+0x535/0xb50
> > [ 275.680881] ? set_termios+0x7d0/0x7d0
> > [ 275.680885] ? check_same_owner+0x320/0x320
> > [ 275.680887] ? graph_lock+0x170/0x170
> > [ 275.680891] n_tty_ioctl_helper+0x54/0x3b0
> > [ 275.680894] n_tty_ioctl+0x54/0x320
> > [ 275.680897] ? ldsem_down_read+0x37/0x40
> > [ 275.680900] ? ldsem_down_read+0x37/0x40
> > [ 275.680903] tty_ioctl+0x5e1/0x1870
> > [ 275.680906] ? commit_echoes+0x1d0/0x1d0
> > [ 275.680909] ? tty_vhangup+0x30/0x30
> > [ 275.680913] ? rcu_is_watching+0x85/0x140
> > [ 275.680916] ? rcu_report_qs_rnp+0x790/0x790
> > [ 275.680919] ? __fget+0x40c/0x650
> > [ 275.680922] ? find_held_lock+0x31/0x1c0
> > [ 275.680926] ? expand_files.part.8+0x9a0/0x9a0
> > [ 275.680929] ? kasan_check_write+0x14/0x20
> > [ 275.680933] ? __mutex_unlock_slowpath+0x180/0x8a0
> > [ 275.680936] ? wait_for_completion+0x870/0x870
> > [ 275.680939] ? tty_vhangup+0x30/0x30
> > [ 275.680942] do_vfs_ioctl+0x1cf/0x16f0
> > [ 275.680945] ? ioctl_preallocate+0x2e0/0x2e0
> > [ 275.680948] ? fget_raw+0x20/0x20
> > [ 275.680951] ? __sb_end_write+0xac/0xe0
> > [ 275.680955] ? __sanitizer_cov_trace_const_cmp1+0x1a/0x20
> > [ 275.680958] ? fput+0x130/0x1a0
> > [ 275.680961] ? ksys_write+0x1a6/0x250
> > [ 275.680965] ? security_file_ioctl+0x94/0xc0
> > [ 275.680968] ksys_ioctl+0xa9/0xd0
> > [ 275.680970] __x64_sys_ioctl+0x73/0xb0
> > [ 275.680974] do_syscall_64+0x1b1/0x800
> > [ 275.680977] ? finish_task_switch+0x1ca/0x840
> > [ 275.680981] ? syscall_return_slowpath+0x5c0/0x5c0
> > [ 275.680984] ? syscall_return_slowpath+0x30f/0x5c0
> > [ 275.680988] ? entry_SYSCALL_64_after_hwframe+0x59/0xbe
> > [ 275.680992] ? trace_hardirqs_off_thunk+0x1a/0x1c
> > [ 275.680996] entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > [ 275.680999] RIP: 0033:0x4559f9
> > [ 275.681001] Code: 1d ba fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff
> >
> > >> /*
> > >> @@ -773,7 +774,13 @@ static inline void proc_tty_unregister_driver(struct tty_driver *d) {}
> > >> #endif
> > >>
> > >> #define tty_msg(fn, tty, f, ...) \
> > >> - fn("%s %s: " f, tty_driver_name(tty), tty_name(tty), ##__VA_ARGS__)
> > >> + do { \
> > >> + unsigned long flags; \
> > >> + \
> > >> + printk_safe_enter_irqsave(flags); \
> > >> + fn("%s %s: " f, tty_driver_name(tty), tty_name(tty), ##__VA_ARGS__); \
> > >> + printk_safe_exit_irqrestore(flags); \
> > >> + } while (0)
> > >
> > > There is no need to call single printk() in printk_safe() context.
> > > printk_deferred() is enough.
> >
> > Then, regarding tty_msg() we can update
> >
> > #define tty_msg(fn, tty, f, ...) \
> > fn("%s %s: " f, tty_driver_name(tty), tty_name(tty), ##__VA_ARGS__)
> >
> > #define tty_debug(tty, f, ...) tty_msg(pr_debug, tty, f, ##__VA_ARGS__)
> > #define tty_info(tty, f, ...) tty_msg(pr_info, tty, f, ##__VA_ARGS__)
> > #define tty_notice(tty, f, ...) tty_msg(pr_notice, tty, f, ##__VA_ARGS__)
> > #define tty_warn(tty, f, ...) tty_msg(pr_warn, tty, f, ##__VA_ARGS__)
> > #define tty_err(tty, f, ...) tty_msg(pr_err, tty, f, ##__VA_ARGS__)
> >
> > #define tty_info_ratelimited(tty, f, ...) \
> > tty_msg(pr_info_ratelimited, tty, f, ##__VA_ARGS__)
> >
> > to use printk_deferred().
> >
> > > Just note that printk_deferred()/printk_safe() is only temporary
> > > solution. It will get obsoleted by offloading the console handling
> > > to kthreads. It goes slowly because it is not trivial. We want
> > > to do it a clean way and carefully. Any historic or new
> > > workarounds/hacks would only slow it down.
> > >
> > > My estimation is that we could have the offloaded consoles
> > > within the next 3-4 development cycles.
> > >
> > > Please, consider whether it is really worth adding these workarounds.
> >
> > 1000+ days have elapsed since this problem was reported. Given that
> > offloading the console handling to kthreads needs big changes which
> > are not suitable for backporting, what can we offer for current and
> > stable kernels?
Note that any deadlocks caused by printk were invisible for lockdep before
the commit f975237b76827956fe13ecfe9 ("printk: use printk_safe buffers
in printk"). It appeared in 4.11-rc1.
Some reported deadlocks are possible in reality but pretty hard to hit.
There are also many false positives caused by printk() messages in
the init code. They can't cause deadlock because the messages are
printed before the given console is registered.
I see three possible solutions:
1. Disable the lockdep in printk() and hide all these problems.
2. Do end-less number of hacks in the console drivers code to
calm down these reports.
3. Keep the status quo and focus on doing the console offload
properly.
We are working hard on the rework. Note that offloading console
work is being discussed minimally since 2013, see
https://lore.kernel.org/lkml/1358272714-23375-1-git-send-email-jack@suse.cz/
And I think that we are really getting close now. I believe that
we will have it on 90% this year.
I am not fully comfortable with disabling the lockdep. We accepted
some fixes because we considered the problems important enough.
IMHO, the workarounds are not always worth it. Some proposed patches
shuffled the locking or printk() and made the code more tricky.
printk_deferred() or printk_safe context are less evil. But they would
need to get reverted once the printk rework is done.
It might be acceptable to use printk_deferred/printk_safe on few
most visible code paths. But I would really like to avoid storm
of patches adding them all over the kernel.
> As it can't be triggered without fault injections, is this even a "real"
> issue? Let's let the printk developers get this right.
+1
Best Regards,
Petr
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