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.
Fix it by checking the NMI backtrace bit in the panic_print prior to the CPU disabling function.
Fixes: 8d470a45d1a6 ("panic: add option to dump all CPUs backtraces in panic_print") Cc: stable@vger.kernel.org Signed-off-by: Guilherme G. Piccoli gpiccoli@igalia.com
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V4: - Sent as standalone patch, rebased against v6.2-rc7.
V2 / V3: - New patch, there was no V1 of this one. Link for V3: https://lore.kernel.org/lkml/20220819221731.480795-12-gpiccoli@igalia.com/
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.
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(); - if (panic_print & PANIC_PRINT_TASK_INFO) show_state();
@@ -243,6 +240,30 @@ void check_panic_on_warn(const char *origin) origin, limit); }
+/* + * Helper that triggers the NMI backtrace (if set in panic_print) + * and then performs the secondary CPUs shutdown - we cannot have + * the NMI backtrace after the CPUs are off! + */ +static void panic_other_cpus_shutdown(void) +{ + if (panic_print & PANIC_PRINT_ALL_CPU_BT) + trigger_all_cpu_backtrace(); + + /* + * Note that smp_send_stop() is the usual SMP shutdown function, + * which unfortunately may not be hardened to work in a panic + * situation. If we want to do crash dump after notifier calls + * and kmsg_dump, we will need architecture dependent extra + * bits in addition to stopping other CPUs, hence we rely on + * crash_smp_send_stop() for that. + */ + if (!crash_kexec_post_notifiers) + smp_send_stop(); + else + crash_smp_send_stop(); +} + /** * panic - halt the system * @fmt: The text string to print @@ -258,7 +279,6 @@ void panic(const char *fmt, ...) long i, i_next = 0, len; int state = 0; int old_cpu, this_cpu; - bool _crash_kexec_post_notifiers = crash_kexec_post_notifiers;
if (panic_on_warn) { /* @@ -333,23 +353,10 @@ 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);
- /* - * Note smp_send_stop is the usual smp shutdown function, which - * unfortunately means it may not be hardened to work in a - * panic situation. - */ - smp_send_stop(); - } else { - /* - * If we want to do crash dump after notifier calls and - * kmsg_dump, we will need architecture dependent extra - * works in addition to stopping other CPUs. - */ - crash_smp_send_stop(); - } + panic_other_cpus_shutdown();
/* * Run any panic handlers, including those that might need to @@ -370,7 +377,7 @@ 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);
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