On Fri 2023-02-10 17:35:10, Guilherme G. Piccoli wrote:
Commit 8d470a45d1a6 ("panic: add option to dump all CPUs backtraces in panic_print") introduced a setting for the "panic_print" kernel parameter to allow users to request a NMI backtrace on panic. Problem is that the panic_print handling happens after the secondary CPUs are already disabled, hence this option ended-up being kind of a no-op - kernel skips the NMI trace in idling CPUs, which is the case of offline CPUs.
Great catch!
Hi folks, thanks in advance for reviews/comments.
Notice that while at it, I got rid of the "crash_kexec_post_notifiers" local copy in panic(). This was introduced by commit b26e27ddfd2a ("kexec: use core_param for crash_kexec_post_notifiers boot option"), but it is not clear from comments or commit message why this local copy is required.
My understanding is that it's a mechanism to prevent some concurrency, in case some other CPU modify this variable while panic() is running. I find it very unlikely, hence I removed it - but if people consider this copy needed, I can respin this patch and keep it, even providing a comment about that, in order to be explict about its need.
Yes, I think that it makes the behavior consistent even when the global variable manipulated in parallel.
I would personally prefer to keep the local copy. Better safe than sorry.
Let me know your thoughts! Cheers,
Guilherme
kernel/panic.c | 47 +++++++++++++++++++++++++++-------------------- 1 file changed, 27 insertions(+), 20 deletions(-)
diff --git a/kernel/panic.c b/kernel/panic.c index 463c9295bc28..f45ee88be8a2 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -211,9 +211,6 @@ static void panic_print_sys_info(bool console_flush) return; }
- if (panic_print & PANIC_PRINT_ALL_CPU_BT)
trigger_all_cpu_backtrace();
Sigh, this is yet another PANIC_PRINT_ action that need special timing. We should handle both the same way.
What about the following? The parameter @mask says what actions are allowed at the given time.
--- a/kernel/panic.c +++ b/kernel/panic.c @@ -72,6 +72,9 @@ EXPORT_SYMBOL_GPL(panic_timeout); #define PANIC_PRINT_FTRACE_INFO 0x00000010 #define PANIC_PRINT_ALL_PRINTK_MSG 0x00000020 #define PANIC_PRINT_ALL_CPU_BT 0x00000040 +/* Filter out actions that need special timing. */ +#define PANIC_PRINT_COMMON_INFO_MASK ~(PANIC_PRINT_ALL_PRINTK_MSG | \ + PANIC_PRINT_ALL_CPU_BT) unsigned long panic_print;
ATOMIC_NOTIFIER_HEAD(panic_notifier_list); @@ -203,30 +206,29 @@ void nmi_panic(struct pt_regs *regs, const char *msg) } EXPORT_SYMBOL(nmi_panic);
-static void panic_print_sys_info(bool console_flush) +static void panic_print_sys_info(unsigned long mask) { - if (console_flush) { - if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG) - console_flush_on_panic(CONSOLE_REPLAY_ALL); - return; - } + unsigned long panic_print_now = panic_print & mask; + + if (panic_print_now & PANIC_PRINT_ALL_PRINTK_MSG) + console_flush_on_panic(CONSOLE_REPLAY_ALL);
- if (panic_print & PANIC_PRINT_ALL_CPU_BT) + if (panic_print_now & PANIC_PRINT_ALL_CPU_BT) trigger_all_cpu_backtrace();
- if (panic_print & PANIC_PRINT_TASK_INFO) + if (panic_print_now & PANIC_PRINT_TASK_INFO) show_state();
- if (panic_print & PANIC_PRINT_MEM_INFO) + if (panic_print_now & PANIC_PRINT_MEM_INFO) show_mem(0, NULL);
- if (panic_print & PANIC_PRINT_TIMER_INFO) + if (panic_print_now & PANIC_PRINT_TIMER_INFO) sysrq_timer_list_show();
- if (panic_print & PANIC_PRINT_LOCK_INFO) + if (panic_print_now & PANIC_PRINT_LOCK_INFO) debug_show_all_locks();
- if (panic_print & PANIC_PRINT_FTRACE_INFO) + if (panic_print_now & PANIC_PRINT_FTRACE_INFO) ftrace_dump(DUMP_ALL); }
@@ -333,9 +335,12 @@ void panic(const char *fmt, ...) * * Bypass the panic_cpu check and call __crash_kexec directly. */ - if (!_crash_kexec_post_notifiers) { + if (!_crash_kexec_post_notifiers) __crash_kexec(NULL);
+ panic_print_sys_info(PANIC_PRINT_ALL_CPU_BT); + + if (!_crash_kexec_post_notifiers) { /* * Note smp_send_stop is the usual smp shutdown function, which * unfortunately means it may not be hardened to work in a @@ -357,7 +362,7 @@ void panic(const char *fmt, ...) */ atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
- panic_print_sys_info(false); + panic_print_sys_info(PANIC_PRINT_COMMON_INFO_MASK);
kmsg_dump(KMSG_DUMP_PANIC);
@@ -386,7 +391,7 @@ void panic(const char *fmt, ...) debug_locks_off(); console_flush_on_panic(CONSOLE_FLUSH_PENDING);
- panic_print_sys_info(true); + panic_print_sys_info(PANIC_PRINT_ALL_PRINTK_MSG);
if (!panic_blink) panic_blink = no_blink;
Best Regards, Petr