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When setting 1 to sysfs debug_force_rr_cpu can trigger the following LOCKDEP WARNING. Reproducer: # ssh localhost // to use pty # echo 1 > /sys/module/workqueue/parameters/debug_force_rr_cpu This problem is similar to the one which syzkaller previously found and it had been discussed in the thread: - https://lore.kernel.org/lkml/20180607051019.GA10406@jagdpanzerIV/T/#ec7d3d2bdb8ccdf0f063fd2e5b2a4d8fd45d8d9e5 Fundamental solution for the problem will be the on going printk() rework: - https://lore.kernel.org/lkml/20190607162349.18199-1-john.ogness@linutronix.de/ WARNING: [ 42.605235] ====================================================== [ 42.605236] WARNING: possible circular locking dependency detected [ 42.605237] 5.2.0+ #63 Tainted: G U [ 42.605238] ------------------------------------------------------ [ 42.605239] sysctl_fuzzer/2697 is trying to acquire lock: [ 42.605239] 00000000d28828ac (console_owner){-.-.}, at: console_unlock+0x23f/0x9a0 [ 42.605243] [ 42.605243] but task is already holding lock: [ 42.605244] 00000000f7c4cf7b (&(&port->lock)->rlock){-.-.}, at: pty_write+0xd5/0x1e0 [ 42.605247] [ 42.605248] which lock already depends on the new lock. [ 42.605248] [ 42.605249] [ 42.605250] the existing dependency chain (in reverse order) is: [ 42.605250] [ 42.605251] -> #2 (&(&port->lock)->rlock){-.-.}: [ 42.605254] _raw_spin_lock_irqsave+0x44/0x90 [ 42.605255] tty_port_tty_get+0x24/0x90 [ 42.605255] tty_port_default_wakeup+0x10/0x30 [ 42.605256] tty_port_tty_wakeup+0x5c/0x70 [ 42.605257] uart_write_wakeup+0x3c/0x50 [ 42.605258] serial8250_tx_chars+0x3d2/0xb10 [ 42.605259] serial8250_handle_irq.part.0+0x14a/0x240 [ 42.605259] serial8250_default_handle_irq+0x8c/0xf0 [ 42.605260] serial8250_interrupt+0xd6/0x150 [ 42.605261] __handle_irq_event_percpu+0x1c2/0x640 [ 42.605262] handle_irq_event_percpu+0x71/0x150 [ 42.605263] handle_irq_event+0xa7/0x134 [ 42.605263] handle_edge_irq+0x218/0xa50 [ 42.605264] handle_irq+0x46/0x60 [ 42.605265] do_IRQ+0xbc/0x250 [ 42.605265] ret_from_intr+0x0/0x1d [ 42.605266] default_idle+0x2f/0x2d0 [ 42.605267] arch_cpu_idle+0x15/0x20 [ 42.605268] default_idle_call+0x40/0x60 [ 42.605268] do_idle+0x315/0x3c0 [ 42.605269] cpu_startup_entry+0x20/0x22 [ 42.605270] rest_init+0x1ea/0x333 [ 42.605270] arch_call_rest_init+0xe/0x1b [ 42.605271] start_kernel+0x6e0/0x71b [ 42.605272] x86_64_start_reservations+0x24/0x26 [ 42.605273] x86_64_start_kernel+0x75/0x79 [ 42.605273] secondary_startup_64+0xa4/0xb0 [ 42.605274] [ 42.605275] -> #1 (&port_lock_key){-.-.}: [ 42.605277] _raw_spin_lock_irqsave+0x44/0x90 [ 42.605278] serial8250_console_write+0x142/0x730 [ 42.605279] univ8250_console_write+0x6c/0xa0 [ 42.605280] console_unlock+0x5cf/0x9a0 [ 42.605280] register_console+0x559/0xa10 [ 42.605281] univ8250_console_init+0x23/0x2d [ 42.605282] console_init+0x339/0x4b9 [ 42.605283] start_kernel+0x43d/0x71b [ 42.605283] x86_64_start_reservations+0x24/0x26 [ 42.605284] x86_64_start_kernel+0x75/0x79 [ 42.605285] secondary_startup_64+0xa4/0xb0 [ 42.605286] [ 42.605286] -> #0 (console_owner){-.-.}: [ 42.605289] __lock_acquire+0x2921/0x56a0 [ 42.605290] lock_acquire+0x151/0x380 [ 42.605290] console_unlock+0x29c/0x9a0 [ 42.605291] vprintk_emit+0xee/0x2b0 [ 42.605292] vprintk_default+0x1f/0x30 [ 42.605292] vprintk_func+0x50/0x1df [ 42.605293] printk+0xb2/0xe3 [ 42.605294] __queue_work.cold+0x79/0xc3 [ 42.605295] queue_work_on+0x72/0x80 [ 42.605295] tty_flip_buffer_push+0xbf/0x120 [ 42.605296] pty_write+0x164/0x1e0 [ 42.605297] n_tty_write+0x368/0x10e0 [ 42.605298] tty_write+0x381/0x710 [ 42.605298] __vfs_write+0x65/0x110 [ 42.605299] vfs_write+0x171/0x4b0 [ 42.605300] ksys_write+0x122/0x220 [ 42.605301] __x64_sys_write+0x73/0xb0 [ 42.605302] do_syscall_64+0x9a/0x450 [ 42.605303] entry_SYSCALL_64_after_hwframe+0x49/0xbe [ 42.605304] [ 42.605305] other info that might help us debug this: [ 42.605306] [ 42.605307] Chain exists of: [ 42.605308] console_owner --> &port_lock_key --> &(&port->lock)->rlock [ 42.605314] [ 42.605316] Possible unsafe locking scenario: [ 42.605316] [ 42.605318] CPU0 CPU1 [ 42.605319] ---- ---- [ 42.605320] lock(&(&port->lock)->rlock); [ 42.605323] lock(&port_lock_key); [ 42.605326] lock(&(&port->lock)->rlock); [ 42.605329] lock(console_owner); [ 42.605332] [ 42.605334] *** DEADLOCK *** [ 42.605335] [ 42.605336] 7 locks held by sysctl_fuzzer/2697: [ 42.605337] #0: 0000000060a0e6d1 (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x33/0x40 [ 42.605343] #1: 0000000038316e97 (&tty->atomic_write_lock){+.+.}, at: tty_write_lock+0x1d/0x50 [ 42.605348] #2: 000000000e2d59d0 (&o_tty->termios_rwsem/1){++++}, at: n_tty_write+0x17e/0x10e0 [ 42.605356] #3: 0000000039ef394c (&ldata->output_lock){+.+.}, at: n_tty_write+0x44c/0x10e0 [ 42.605361] #4: 00000000f7c4cf7b (&(&port->lock)->rlock){-.-.}, at: pty_write+0xd5/0x1e0 [ 42.605364] #5: 0000000017933ff3 (rcu_read_lock){....}, at: __queue_work+0xeb/0x1000 [ 42.605367] #6: 0000000088e0121a (console_lock){+.+.}, at: vprintk_emit+0xe5/0x2b0 [ 42.605370] [ 42.605371] stack backtrace: [ 42.605372] CPU: 1 PID: 2697 Comm: sysctl_fuzzer Tainted: G U 5.2.0+ #63 [ 42.605373] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-2.fc30 04/01/2014 [ 42.605374] Call Trace: [ 42.605374] dump_stack+0x86/0xca [ 42.605375] print_circular_bug.cold+0x163/0x172 [ 42.605376] check_noncircular+0x330/0x3d0 [ 42.605376] ? print_circular_bug+0x1b0/0x1b0 [ 42.605377] ? graph_lock+0x7a/0x180 [ 42.605378] ? check_chain_key+0x590/0x590 [ 42.605379] __lock_acquire+0x2921/0x56a0 [ 42.605379] ? register_lock_class+0x1640/0x1640 [ 42.605380] lock_acquire+0x151/0x380 [ 42.605381] ? console_unlock+0x23f/0x9a0 [ 42.605382] console_unlock+0x29c/0x9a0 [ 42.605382] ? console_unlock+0x23f/0x9a0 [ 42.605383] ? __down_trylock_console_sem+0x89/0xa0 [ 42.605384] vprintk_emit+0xee/0x2b0 [ 42.605384] vprintk_default+0x1f/0x30 [ 42.605385] vprintk_func+0x50/0x1df [ 42.605386] ? debug_object_activate+0x20e/0x490 [ 42.605387] printk+0xb2/0xe3 [ 42.605387] ? kmsg_dump_rewind_nolock+0xe4/0xe4 [ 42.605388] ? lock_acquire+0x151/0x380 [ 42.605389] ? __queue_work+0xeb/0x1000 [ 42.605389] ? kasan_check_read+0x11/0x20 [ 42.605390] ? rcu_dynticks_curr_cpu_in_eqs+0x87/0x100 [ 42.605391] __queue_work.cold+0x79/0xc3 [ 42.605392] ? lockdep_hardirqs_off+0x1f4/0x2e0 [ 42.605392] queue_work_on+0x72/0x80 [ 42.605393] tty_flip_buffer_push+0xbf/0x120 [ 42.605394] pty_write+0x164/0x1e0 [ 42.605395] n_tty_write+0x368/0x10e0 [ 42.605395] ? n_tty_open+0x160/0x160 [ 42.605396] ? do_wait_intr_irq+0x350/0x350 [ 42.605397] ? kasan_check_write+0x14/0x20 [ 42.605397] tty_write+0x381/0x710 [ 42.605398] ? n_tty_open+0x160/0x160 [ 42.605399] __vfs_write+0x65/0x110 [ 42.605399] vfs_write+0x171/0x4b0 [ 42.605400] ksys_write+0x122/0x220 [ 42.605401] ? __ia32_sys_read+0xb0/0xb0 [ 42.605402] ? entry_SYSCALL_64_after_hwframe+0x49/0xbe [ 42.605402] ? lockdep_hardirqs_on+0x35b/0x5d0 [ 42.605403] __x64_sys_write+0x73/0xb0 [ 42.605404] do_syscall_64+0x9a/0x450 [ 42.605405] entry_SYSCALL_64_after_hwframe+0x49/0xbe [ 42.605405] RIP: 0033:0x7fe9c296d0f8 [ 42.605407] Code: 89 02 48 c7 c0 ff ff ff ff eb bb 0f 1f 80 00 00 00 00 f3 0f 1e fa 48 8d 05 25 96 0d 00 8b 00 85 c0 75 17 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 60 c3 0f 1f 80 00 00 00 00 48 83 ec 28 48 89 [ 42.605408] RSP: 002b:00007fff1abc6f68 EFLAGS: 00000246 ORIG_RAX: 0000000000000001 [ 42.605409] RAX: ffffffffffffffda RBX: 000000000000003a RCX: 00007fe9c296d0f8 [ 42.605410] RDX: 000000000000003a RSI: 00007fff1abc7140 RDI: 0000000000000001 [ 42.605411] RBP: 00007fff1abc7140 R08: 00007fe9c2a48500 R09: 000000000000003a [ 42.605412] R10: 000000000051c920 R11: 0000000000000246 R12: 000000000000003a [ 42.605414] R13: 00007fe9c2a42780 R14: 000000000000003a R15: 00007fe9c2a3d740 [ 42.792367] ------------[ cut here ]------------