In 2019, Sergey fixed a lockdep splat with 15341b1dd409 ("char/random: silence a lockdep splat with printk()"), but that got reverted soon after from 4.19 because back then it apparently caused various problems. But the issue it was fixing is still there, and more generally, many patches turning printk() into printk_deferred() have landed since, making me suspect it's okay to try this out again.

This should fix the following deadlock found by the kernel test robot:

[ 18.287691] WARNING: possible circular locking dependency detected [ 18.287692] 4.19.248-00165-g3d1f971aa81f #1 Not tainted [ 18.287693] ------------------------------------------------------ [ 18.287712] stop/202 is trying to acquire lock: [ 18.287713] (ptrval) (console_owner){..-.}, at: console_unlock (??:?) [ 18.287717] [ 18.287718] but task is already holding lock: [ 18.287718] (ptrval) (&(&port->lock)->rlock){-...}, at: pty_write (pty.c:?) [ 18.287722] [ 18.287722] which lock already depends on the new lock. [ 18.287723] [ 18.287724] [ 18.287725] the existing dependency chain (in reverse order) is: [ 18.287725] [ 18.287726] -> #2 (&(&port->lock)->rlock){-...}: [ 18.287729] validate_chain+0x84a/0xe00 [ 18.287729] __lock_acquire (lockdep.c:?) [ 18.287730] lock_acquire (??:?) [ 18.287731] _raw_spin_lock_irqsave (??:?) [ 18.287732] tty_port_tty_get (??:?) [ 18.287733] tty_port_default_wakeup (tty_port.c:?) [ 18.287734] tty_port_tty_wakeup (??:?) [ 18.287734] uart_write_wakeup (??:?) [ 18.287735] serial8250_tx_chars (??:?) [ 18.287736] serial8250_handle_irq (??:?) [ 18.287737] serial8250_default_handle_irq (8250_port.c:?) [ 18.287738] serial8250_interrupt (8250_core.c:?) [ 18.287738] __handle_irq_event_percpu (??:?) [ 18.287739] handle_irq_event_percpu (??:?) [ 18.287740] handle_irq_event (??:?) [ 18.287741] handle_edge_irq (??:?) [ 18.287742] handle_irq (??:?) [ 18.287742] do_IRQ (??:?) [ 18.287743] common_interrupt (entry_32.o:?) [ 18.287744] _raw_spin_unlock_irqrestore (??:?) [ 18.287745] uart_write (serial_core.c:?) [ 18.287746] process_output_block (n_tty.c:?) [ 18.287747] n_tty_write (n_tty.c:?) [ 18.287747] tty_write (tty_io.c:?) [ 18.287748] __vfs_write (??:?) [ 18.287749] vfs_write (??:?) [ 18.287750] ksys_write (??:?) [ 18.287750] sys_write (??:?) [ 18.287751] do_fast_syscall_32 (??:?) [ 18.287752] entry_SYSENTER_32 (??:?) [ 18.287752] [ 18.287753] -> #1 (&port_lock_key){-.-.}: [ 18.287756] [ 18.287756] -> #0 (console_owner){..-.}: [ 18.287759] check_prevs_add (lockdep.c:?) [ 18.287760] validate_chain+0x84a/0xe00 [ 18.287761] __lock_acquire (lockdep.c:?) [ 18.287761] lock_acquire (??:?) [ 18.287762] console_unlock (??:?) [ 18.287763] vprintk_emit (??:?) [ 18.287764] vprintk_default (??:?) [ 18.287764] vprintk_func (??:?) [ 18.287765] printk (??:?) [ 18.287766] get_random_u32 (??:?) [ 18.287767] shuffle_freelist (slub.c:?) [ 18.287767] allocate_slab (slub.c:?) [ 18.287768] new_slab (slub.c:?) [ 18.287769] ___slab_alloc+0x6d0/0xb20 [ 18.287770] __slab_alloc+0xd6/0x2e0 [ 18.287770] __kmalloc (??:?) [ 18.287771] tty_buffer_alloc (tty_buffer.c:?) [ 18.287772] __tty_buffer_request_room (tty_buffer.c:?) [ 18.287773] tty_insert_flip_string_fixed_flag (??:?) [ 18.287774] pty_write (pty.c:?) [ 18.287775] process_output_block (n_tty.c:?) [ 18.287776] n_tty_write (n_tty.c:?) [ 18.287777] tty_write (tty_io.c:?) [ 18.287778] __vfs_write (??:?) [ 18.287779] vfs_write (??:?) [ 18.287780] ksys_write (??:?) [ 18.287780] sys_write (??:?) [ 18.287781] do_fast_syscall_32 (??:?) [ 18.287782] entry_SYSENTER_32 (??:?) [ 18.287783] [ 18.287783] other info that might help us debug this: [ 18.287784] [ 18.287785] Chain exists of: [ 18.287785] console_owner --> &port_lock_key --> &(&port->lock)->rlock [ 18.287789] [ 18.287790] Possible unsafe locking scenario: [ 18.287790] [ 18.287791] CPU0 CPU1 [ 18.287792] ---- ---- [ 18.287792] lock(&(&port->lock)->rlock); [ 18.287794] lock(&port_lock_key); [ 18.287814] lock(&(&port->lock)->rlock); [ 18.287815] lock(console_owner); [ 18.287817] [ 18.287818] *** DEADLOCK *** [ 18.287818] [ 18.287819] 6 locks held by stop/202: [ 18.287820] #0: (ptrval) (&tty->ldisc_sem){++++}, at: ldsem_down_read (??:?) [ 18.287823] #1: (ptrval) (&tty->atomic_write_lock){+.+.}, at: tty_write_lock (tty_io.c:?) [ 18.287826] #2: (ptrval) (&o_tty->termios_rwsem/1){++++}, at: n_tty_write (n_tty.c:?) [ 18.287830] #3: (ptrval) (&ldata->output_lock){+.+.}, at: process_output_block (n_tty.c:?) [ 18.287834] #4: (ptrval) (&(&port->lock)->rlock){-...}, at: pty_write (pty.c:?) [ 18.287838] #5: (ptrval) (console_lock){+.+.}, at: console_trylock_spinning (printk.c:?) [ 18.287841] [ 18.287842] stack backtrace: [ 18.287843] CPU: 0 PID: 202 Comm: stop Not tainted 4.19.248-00165-g3d1f971aa81f #1 [ 18.287843] Call Trace: [ 18.287844] dump_stack (??:?) [ 18.287845] print_circular_bug.cold+0x78/0x8b [ 18.287846] check_prev_add+0x66a/0xd20 [ 18.287847] check_prevs_add (lockdep.c:?) [ 18.287848] validate_chain+0x84a/0xe00 [ 18.287848] __lock_acquire (lockdep.c:?) [ 18.287849] lock_acquire (??:?) [ 18.287850] ? console_unlock (??:?) [ 18.287851] console_unlock (??:?) [ 18.287851] ? console_unlock (??:?) [ 18.287852] ? native_save_fl (??:?) [ 18.287853] vprintk_emit (??:?) [ 18.287854] vprintk_default (??:?) [ 18.287855] vprintk_func (??:?) [ 18.287855] printk (??:?) [ 18.287856] get_random_u32 (??:?) [ 18.287857] ? shuffle_freelist (slub.c:?) [ 18.287858] shuffle_freelist (slub.c:?) [ 18.287858] ? page_address (??:?) [ 18.287859] allocate_slab (slub.c:?) [ 18.287860] new_slab (slub.c:?) [ 18.287861] ? pvclock_clocksource_read (??:?) [ 18.287862] ___slab_alloc+0x6d0/0xb20 [ 18.287862] ? kvm_sched_clock_read (kvmclock.c:?) [ 18.287863] ? __slab_alloc+0xbc/0x2e0 [ 18.287864] ? native_wbinvd (paravirt.c:?) [ 18.287865] __slab_alloc+0xd6/0x2e0 [ 18.287865] __kmalloc (??:?) [ 18.287866] ? __lock_acquire (lockdep.c:?) [ 18.287867] ? tty_buffer_alloc (tty_buffer.c:?) [ 18.287868] tty_buffer_alloc (tty_buffer.c:?) [ 18.287869] __tty_buffer_request_room (tty_buffer.c:?) [ 18.287869] tty_insert_flip_string_fixed_flag (??:?) [ 18.287870] pty_write (pty.c:?) [ 18.287871] process_output_block (n_tty.c:?) [ 18.287872] n_tty_write (n_tty.c:?) [ 18.287873] ? print_dl_stats (??:?) [ 18.287874] ? n_tty_ioctl (n_tty.c:?) [ 18.287874] tty_write (tty_io.c:?) [ 18.287875] ? n_tty_ioctl (n_tty.c:?) [ 18.287876] ? tty_write_unlock (tty_io.c:?) [ 18.287877] __vfs_write (??:?) [ 18.287877] vfs_write (??:?) [ 18.287878] ? __fget_light (file.c:?) [ 18.287879] ksys_write (??:?)

Cc: Sergey Senozhatsky sergey.senozhatsky.work@gmail.com Cc: Qian Cai cai@lca.pw Cc: Lech Perczak l.perczak@camlintechnologies.com Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Theodore Ts'o tytso@mit.edu Cc: Sasha Levin sashal@kernel.org Cc: Petr Mladek pmladek@suse.com Cc: John Ogness john.ogness@linutronix.de Reported-by: kernel test robot oliver.sang@intel.com Link: https://lore.kernel.org/lkml/Ytz+lo4zRQYG3JUR@xsang-OptiPlex-9020 Signed-off-by: Jason A. Donenfeld Jason@zx2c4.com --- drivers/char/random.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c index bac4dc501dc4..2be38780a7f7 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -183,8 +183,8 @@ static void __cold process_random_ready_list(void)

#define warn_unseeded_randomness() \ if (IS_ENABLED(CONFIG_WARN_ALL_UNSEEDED_RANDOM) && !crng_ready()) \ - pr_notice("%s called from %pS with crng_init=%d\n", \ - __func__, (void *)_RET_IP_, crng_init) + printk_deferred(KERN_NOTICE "random: %s called from %pS with crng_init=%d\n", \ + __func__, (void *)_RET_IP_, crng_init)

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