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Petr Mladek c41c0ebfa1 printk/nbcon: Block printk kthreads when any CPU is in an emergency context 
In emergency contexts, printk() tries to flush messages directly even
on nbcon consoles. And it is allowed to takeover the console ownership
and interrupt the printk kthread in the middle of a message.

Only one takeover and one repeated message should be enough in most
situations. The first emergency message flushes the backlog and printk
kthreads get to sleep. Next emergency messages are flushed directly
and printk() does not wake up the kthreads.

However, the one takeover is not guaranteed. Any printk() in normal
context on another CPU could wake up the kthreads. Or a new emergency
message might be added before the kthreads get to sleep. Note that
the interrupted .write_thread() callbacks usually have to call
nbcon_reacquire_nobuf() and restore the original device setting
before checking for pending messages.

The risk of the repeated takeovers will be even bigger because
__nbcon_atomic_flush_pending_con is going to release the console
ownership after each emitted record. It will be needed to prevent
hardlockup reports on other CPUs which are busy waiting for
the context ownership, for example, by nbcon_reacquire_nobuf() or
__uart_port_nbcon_acquire().

The repeated takeovers break the output, for example:

    [ 5042.650211][ T2220] Call Trace:
    [ 5042.6511
    ** replaying previous printk message **
    [ 5042.651192][ T2220]  <TASK>
    [ 5042.652160][ T2220]  kunit_run_
    ** replaying previous printk message **
    [ 5042.652160][ T2220]  kunit_run_tests+0x72/0x90
    [ 5042.653340][ T22
    ** replaying previous printk message **
    [ 5042.653340][ T2220]  ? srso_alias_return_thunk+0x5/0xfbef5
    [ 5042.654628][ T2220]  ? stack_trace_save+0x4d/0x70
    [ 5042.6553
    ** replaying previous printk message **
    [ 5042.655394][ T2220]  ? srso_alias_return_thunk+0x5/0xfbef5
    [ 5042.656713][ T2220]  ? save_trace+0x5b/0x180

A more robust solution is to block the printk kthread entirely whenever
*any* CPU enters an emergency context. This ensures that critical messages
can be flushed without contention from the normal, non-atomic printing
path.

Link: https://lore.kernel.org/all/aNQO-zl3k1l4ENfy@pathway.suse.cz
Reviewed-by: Andrew Murray <amurray@thegoodpenguin.co.uk>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20250926124912.243464-2-pmladek@suse.com
[pmladek@suse.com: Added changes proposed by John Ogness]
Signed-off-by: Petr Mladek <pmladek@suse.com>
2025-10-30 12:10:16 +01:00
..
bpf bpf-fixes 2025-10-03 19:38:19 -07:00
cgroup bpf-next-6.18 2025-09-30 17:58:11 -07:00
configs kcfi: Rename CONFIG_CFI_CLANG to CONFIG_CFI 2025-09-24 14:29:14 -07:00
debug kdb: remove redundant check for scancode 0xe0 2025-09-20 21:19:09 +01:00
dma dma-mapping updates for Linux 6.18: 2025-10-03 17:41:12 -07:00
entry entry: Add arch_irqentry_exit_need_resched() for arm64 2025-09-11 15:55:34 +01:00
events Summary of significant series in this pull request: 2025-10-02 18:18:33 -07:00
futex A set of updates for futexes and related selftests: 2025-09-30 16:07:10 -07:00
gcov kbuild: require gcc-8 and binutils-2.30 2025-04-30 21:53:35 +02:00
irq A set of updates for the interrupt core subsystem: 2025-09-30 15:55:25 -07:00
kcsan Kernel Concurrency Sanitizer (KCSAN) updates for v6.18 2025-10-02 08:31:44 -07:00
livepatch sched,livepatch: Untangle cond_resched() and live-patching 2025-05-14 13:16:24 +02:00
locking locking/local_lock: Introduce local_lock_is_locked(). 2025-09-29 09:42:35 +02:00
module kcfi: Rename CONFIG_CFI_CLANG to CONFIG_CFI 2025-09-24 14:29:14 -07:00
power Merge branches 'pm-core', 'pm-runtime' and 'pm-sleep' 2025-09-29 12:54:01 +02:00
printk printk/nbcon: Block printk kthreads when any CPU is in an emergency context 2025-10-30 12:10:16 +01:00
rcu mm: replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion 2025-09-13 16:54:42 -07:00
sched Summary of significant series in this pull request: 2025-10-02 18:18:33 -07:00
time Networking changes for 6.18. 2025-10-02 15:17:01 -07:00
trace file->f_path constification 2025-10-03 16:32:36 -07:00
unwind unwind: Finish up unwind when a task exits 2025-07-31 10:20:11 -04:00
.gitignore kheaders: rebuild kheaders_data.tar.xz when a file is modified within a minute 2025-06-24 20:30:37 +09:00
acct.c kernel/acct.c: saner struct file treatment 2025-09-27 20:13:56 -04:00
async.c
audit.c audit: fix skb leak when audit rate limit is exceeded 2025-09-10 19:55:00 -04:00
audit.h audit: create audit_stamp structure 2025-08-30 10:15:28 -04:00
audit_fsnotify.c VFS/audit: introduce kern_path_parent() for audit 2025-09-23 12:37:35 +02:00
audit_tree.c mount-related stuff for this cycle 2025-10-03 10:19:44 -07:00
audit_watch.c VFS/audit: introduce kern_path_parent() for audit 2025-09-23 12:37:35 +02:00
auditfilter.c audit/stable-6.18 PR 20250926 2025-09-30 08:22:16 -07:00
auditsc.c audit: add record for multiple object contexts 2025-08-30 10:15:30 -04:00
backtracetest.c
bounds.c
capability.c capability: Remove unused has_capability 2025-03-07 22:03:09 -06:00
cfi.c cfi: Move BPF CFI types and helpers to generic code 2025-07-31 18:23:53 -07:00
compat.c
configs.c
context_tracking.c context_tracking: Make RCU watch ct_kernel_exit_state() warning 2025-03-04 18:44:29 -08:00
cpu.c cpu: Remove obsolete comment from takedown_cpu() 2025-08-06 22:48:12 +02:00
cpu_pm.c
crash_core.c crash: add KUnit tests for crash_exclude_mem_range 2025-09-13 17:32:55 -07:00
crash_core_test.c crash: add KUnit tests for crash_exclude_mem_range 2025-09-13 17:32:55 -07:00
crash_dump_dm_crypt.c crash_dump: retrieve dm crypt keys in kdump kernel 2025-05-21 10:48:21 -07:00
crash_reserve.c kdump: implement reserve_crashkernel_cma 2025-07-19 19:08:23 -07:00
cred.c copy_process: pass clone_flags as u64 across calltree 2025-09-01 15:31:34 +02:00
delayacct.c delayacct: remove redundant code and adjust indentation 2025-05-27 19:40:33 -07:00
dma.c
elfcorehdr.c
exec_domain.c
exit.c task_stack.h: clean-up stack_not_used() implementation 2025-09-21 14:22:00 -07:00
exit.h
extable.c
fail_function.c
fork.c Patch series in this pull request: 2025-10-02 18:44:54 -07:00
freezer.c mm/oom_kill: thaw the entire OOM victim process 2025-09-21 14:22:35 -07:00
gen_kheaders.sh kheaders: make it possible to override TAR 2025-08-06 10:23:36 +09:00
groups.c
hung_task.c hung_task: dump blocker task if it is not hung 2025-09-13 17:32:43 -07:00
iomem.c mm/memremap: Pass down MEMREMAP_* flags to arch_memremap_wb() 2025-02-21 15:05:38 +01:00
irq_work.c kasan: make kasan_record_aux_stack_noalloc() the default behaviour 2025-01-13 22:40:36 -08:00
jump_label.c jump_label: Use RCU in all users of __module_text_address(). 2025-03-10 11:54:46 +01:00
kallsyms.c bpf: Clean up individual BTF_ID code 2025-07-16 18:34:42 -07:00
kallsyms_internal.h
kallsyms_selftest.c kallsyms: use kmalloc_array() instead of kmalloc() 2025-09-28 11:36:14 -07:00
kallsyms_selftest.h
kcmp.c kcmp: improve performance adding an unlikely hint to task comparisons 2025-02-21 10:25:33 +01:00
Kconfig.freezer
Kconfig.hz kernel: Fix "select" wording on HZ_250 description 2025-02-21 09:20:30 +01:00
Kconfig.kexec crash: add KUnit tests for crash_exclude_mem_range 2025-09-13 17:32:55 -07:00
Kconfig.locks
Kconfig.preempt softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT 2025-09-17 16:25:41 +02:00
kcov.c kcov: use write memory barrier after memcpy() in kcov_move_area() 2025-09-13 17:32:44 -07:00
kexec.c kexec: enable CMA based contiguous allocation 2025-08-02 12:01:38 -07:00
kexec_core.c kexec_core: remove redundant 0 value initialization 2025-09-13 17:32:49 -07:00
kexec_elf.c kexec: initialize ELF lowest address to ULONG_MAX 2025-03-16 22:30:47 -07:00
kexec_file.c x86/kexec: carry forward the boot DTB on kexec 2025-09-13 17:32:43 -07:00
kexec_handover.c Patch series in this pull request: 2025-10-02 18:44:54 -07:00
kexec_internal.h kexec: enable CMA based contiguous allocation 2025-08-02 12:01:38 -07:00
kheaders.c
kprobes.c kprobes: Add missing kerneldoc for __get_insn_slot 2025-07-15 18:45:34 +09:00
kstack_erase.c stackleak: Rename stackleak_track_stack to __sanitizer_cov_stack_depth 2025-07-21 21:40:39 -07:00
ksyms_common.c
ksysfs.c kernel/ksysfs.c: simplify bin_attribute definition 2025-01-07 16:59:15 +01:00
kthread.c ipvs: Fix estimator kthreads preferred affinity 2025-08-13 08:34:33 +02:00
latencytop.c treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00
Makefile Patch series in this pull request: 2025-10-02 18:44:54 -07:00
module_signature.c
notifier.c
nscommon.c ns: drop assert 2025-09-25 09:23:54 +02:00
nsproxy.c namespace-6.18-rc1 2025-09-29 11:20:29 -07:00
nstree.c ns: move ns type into struct ns_common 2025-09-25 09:23:54 +02:00
padata.c padata: use cpumask_nth() 2025-06-13 17:26:17 +08:00
panic.c panic: remove CONFIG_PANIC_ON_OOPS_VALUE 2025-09-28 11:36:13 -07:00
params.c params: Replace deprecated strcpy() with strscpy() and memcpy() 2025-08-16 21:47:25 +02:00
pid.c namespace-6.18-rc1 2025-09-29 11:20:29 -07:00
pid_namespace.c namespace-6.18-rc1 2025-09-29 11:20:29 -07:00
pid_sysctl.h treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00
profile.c
ptrace.c ptrace: introduce PTRACE_SET_SYSCALL_INFO request 2025-05-11 17:48:15 -07:00
range.c
reboot.c - The 7 patch series "powerpc/crash: use generic crashkernel 2025-04-01 10:06:52 -07:00
regset.c
relay.c relayfs: support a counter tracking if data is too big to write 2025-07-09 22:57:52 -07:00
resource.c resource: improve child resource handling in release_mem_region_adjustable() 2025-09-21 14:22:34 -07:00
resource_kunit.c
rseq.c rseq: Protect event mask against membarrier IPI 2025-09-13 19:51:59 +02:00
scftorture.c
scs.c
seccomp.c Performance events updates for v6.18: 2025-09-30 11:11:21 -07:00
signal.c signal: Fix memory leak for PIDFD_SELF* sentinels 2025-08-19 13:51:28 +02:00
smp.c smp: Fix up and expand the smp_call_function_many() kerneldoc 2025-09-18 22:21:28 +02:00
smpboot.c sched/smp: Use the SMP version of idle_thread_set_boot_cpu() 2025-06-13 08:47:20 +02:00
smpboot.h
softirq.c softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT 2025-09-17 16:25:41 +02:00
stacktrace.c
static_call.c
static_call_inline.c Modules changes for 6.15-rc1 2025-03-30 15:44:36 -07:00
stop_machine.c sched/core: Fix migrate_swap() vs. hotplug 2025-07-01 15:02:03 +02:00
sys.c Patch series in this pull request: 2025-10-02 18:44:54 -07:00
sys_ni.c uprobes/x86: Add uprobe syscall to speed up uprobe 2025-08-21 20:09:20 +02:00
sysctl-test.c sysctl: move u8 register test to lib/test_sysctl.c 2025-04-14 14:13:41 +02:00
sysctl.c sysctl: rename kern_table -> sysctl_subsys_table 2025-07-23 11:56:02 +02:00
task_work.c kasan: make kasan_record_aux_stack_noalloc() the default behaviour 2025-01-13 22:40:36 -08:00
taskstats.c
torture.c torture: Add get_torture_init_jiffies() for test-start time 2025-02-05 07:14:24 -08:00
tracepoint.c tracepoint: Print the function symbol when tracepoint_debug is set 2025-03-21 15:30:10 -04:00
tsacct.c pid: change bacct_add_tsk() to use task_ppid_nr_ns() 2025-08-19 13:38:20 +02:00
ucount.c ucount: use atomic_long_try_cmpxchg() in atomic_long_inc_below() 2025-08-02 12:01:38 -07:00
uid16.c
uid16.h
umh.c treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00
up.c
user-return-notifier.c
user.c ns: move ns type into struct ns_common 2025-09-25 09:23:54 +02:00
user_namespace.c ns: move ns type into struct ns_common 2025-09-25 09:23:54 +02:00
utsname.c namespace-6.18-rc1 2025-09-29 11:20:29 -07:00
utsname_sysctl.c treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00
vhost_task.c vhost: Take a reference on the task in struct vhost_task. 2025-09-21 17:44:20 -04:00
vmcore_info.c crash: export PAGE_UNACCEPTED_MAPCOUNT_VALUE to vmcoreinfo 2025-05-11 17:54:04 -07:00
watch_queue.c vfs-6.15-rc1.pipe 2025-03-24 09:52:37 -07:00
watchdog.c watchdog: skip checks when panic is in progress 2025-09-13 17:32:53 -07:00
watchdog_buddy.c watchdog: fix opencoded cpumask_next_wrap() in watchdog_next_cpu() 2025-07-31 11:28:03 -04:00
watchdog_perf.c watchdog: skip checks when panic is in progress 2025-09-13 17:32:53 -07:00
workqueue.c workqueue: WQ_PERCPU added to alloc_workqueue users 2025-09-16 10:33:53 -10:00
workqueue_internal.h