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charset="utf-8" Suggested-by: Petr Mladek Signed-off-by: Stephen Brennan --- We cannot define a static inline without including linux/atomic.h, so I just added a macro for convenience in later patches. Since macros were the only option, I didn't include a helper for panic_in_progress_different_cpu(). include/linux/panic.h | 3 +++ 1 file changed, 3 insertions(+) diff --git a/include/linux/panic.h b/include/linux/panic.h index f5844908a089..8e8bd50494d5 100644 --- a/include/linux/panic.h +++ b/include/linux/panic.h @@ -45,6 +45,9 @@ extern bool crash_kexec_post_notifiers; extern atomic_t panic_cpu; #define PANIC_CPU_INVALID -1 =20 +#define panic_in_progress() \ + unlikely(atomic_read(&panic_cpu) !=3D PANIC_CPU_INVALID) + /* * Only to be used by arch init code. 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charset="utf-8" A CPU executing with console lock spinning enabled might be halted during a panic. Before the panicking CPU calls console_flush_on_panic(), it may call console_trylock(), which attempts to optimistically spin, deadlocking the panic CPU: CPU 0 (panic CPU) CPU 1 Suggested-by: Petr Mladek ----------------- ------ printk() { vprintk_func() { vprintk_default() { vprintk_emit() { console_unlock() { console_lock_spinning_enable(); ... printing to console ... panic() { crash_smp_send_stop() { NMI -------------------> HALT } atomic_notifier_call_chain() { printk() { ... console_trylock_spinnning() { // optimistic spin infinitely This hang during panic can be induced when a kdump kernel is loaded, and crash_kexec_post_notifiers=3D1 is present on the kernel command line. The following script which concurrently writes to /dev/kmsg, and triggers a panic, can result in this hang: #!/bin/bash date # 991 chars (based on log buffer size): chars=3D"$(printf 'a%.0s' {1..991})" while :; do echo $chars > /dev/kmsg done & echo c > /proc/sysrq-trigger & date exit To avoid this deadlock, ensure that console_trylock_spinning() does not allow spinning once a panic has begun. Fixes: dbdda842fe96 ("printk: Add console owner and waiter logic to load ba= lance console writes") Suggested-by: Petr Mladek Signed-off-by: Stephen Brennan --- kernel/printk/printk.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 57b132b658e1..20b4b71a1a07 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1843,6 +1843,16 @@ static int console_trylock_spinning(void) if (console_trylock()) return 1; =20 + /* + * It's unsafe to spin once a panic has begun. If we are the + * panic CPU, we may have already halted the owner of the + * console_sem. If we are not the panic CPU, then we should + * avoid taking console_sem, so the panic CPU has a better + * chance of cleanly acquiring it later. + */ + if (panic_in_progress()) + return 0; + printk_safe_enter_irqsave(flags); =20 raw_spin_lock(&console_owner_lock); --=20 2.30.2 From nobody Tue Jun 30 08:12:30 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 35A65C433F5 for ; Fri, 21 Jan 2022 19:03:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231461AbiAUTDE (ORCPT ); Fri, 21 Jan 2022 14:03:04 -0500 Received: from mx0b-00069f02.pphosted.com ([205.220.177.32]:60612 "EHLO mx0b-00069f02.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231483AbiAUTC7 (ORCPT ); Fri, 21 Jan 2022 14:02:59 -0500 Received: from pps.filterd (m0246631.ppops.net [127.0.0.1]) by mx0b-00069f02.pphosted.com (8.16.1.2/8.16.1.2) with SMTP id 20LGwj0a008912; 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charset="utf-8" During a panic(), if another CPU is writing heavily the kernel log (e.g. via /dev/kmsg), then the panic CPU may livelock writing out its messages to the console. Note when too many messages are dropped during panic and suppress further printk, except from the panic CPU. Suggested-by: Petr Mladek Signed-off-by: Stephen Brennan --- kernel/printk/printk.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 20b4b71a1a07..ca253ac07615 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -93,6 +93,12 @@ EXPORT_SYMBOL_GPL(console_drivers); */ int __read_mostly suppress_printk; =20 +/* + * During panic, heavy printk by other CPUs can delay the + * panic and risk deadlock on console resources. + */ +int __read_mostly suppress_panic_printk; + #ifdef CONFIG_LOCKDEP static struct lockdep_map console_lock_dep_map =3D { .name =3D "console_lock" @@ -2228,6 +2234,10 @@ asmlinkage int vprintk_emit(int facility, int level, if (unlikely(suppress_printk)) return 0; =20 + if (unlikely(suppress_panic_printk) && + atomic_read(&panic_cpu) !=3D raw_smp_processor_id()) + return 0; + if (level =3D=3D LOGLEVEL_SCHED) { level =3D LOGLEVEL_DEFAULT; in_sched =3D true; @@ -2613,6 +2623,7 @@ void console_unlock(void) { static char ext_text[CONSOLE_EXT_LOG_MAX]; 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charset="utf-8" If another CPU is in panic, we are about to be halted. Try to gracefully drop console_sem and allow the panic CPU to grab it easily. Suggested-by: Petr Mladek Signed-off-by: Stephen Brennan --- kernel/printk/printk.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index ca253ac07615..c2dc8ebd9509 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2668,7 +2668,7 @@ void console_unlock(void) =20 for (;;) { size_t ext_len =3D 0; - int handover; + int handover, pcpu; size_t len; =20 skip: @@ -2739,6 +2739,12 @@ void console_unlock(void) if (handover) return; =20 + /* Allow panic_cpu to take over the consoles safely */ + pcpu =3D atomic_read(&panic_cpu); + if (unlikely(pcpu !=3D PANIC_CPU_INVALID && + pcpu !=3D raw_smp_processor_id())) + break; + if (do_cond_resched) cond_resched(); } --=20 2.30.2