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
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