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: [email protected]
Signed-off-by: Guilherme G. Piccoli <[email protected]>
---
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/[email protected]/
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);
console_unblank();
--
2.39.1
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
On 14/02/2023 11:46, Petr Mladek wrote:
> [...]
>> 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.
>
Hi Petr, thanks for your review!
OK, we could keep this local copy, makes sense...even adding a comment,
to make its purpose really clear.
>> [...]
>> @@ -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.
> < ..code..>
I think your approach is interesting, it's very "organized".
But I think it's a bit conflicting with that purpose we had on notifiers
refactor, to deprecate "bogus" usages of panic_print, as in
https://lore.kernel.org/lkml/[email protected]/ .
So, the idea of my approach is to allow:
(a) Easy removal of panic_print_sys_info() of panic(), once we move it
to a panic notifier;
(b) Better separate and identify the "bogus" cases. The CPU backtrace
one is less a bogus case in my opinion, more a "complicated" one, since
it's related with the CPUs stop routines. But the console flush, as we
discussed, it's clearly something that calls for a new parameter (and
such param was added in the refactor patch).
In the end, I think your approach is interesting but it's kinda like
we're adding the fix to later, on refactor, entirely remove/rework it.
With my approach we wouldn't be calling panic_print_sys_info() again
(3rd time!) on panic(), and also would be more natural to move it later
to a new panic notifier.
What you / others think? If your approach is in the end preferred, it's
fine by me - I'd just ask you to submit as a full patch so we can get it
merged as a fix in 6.3, if possible (and backport it to the 6.1/6.2
stable). Now, if my approach is fine, I can resubmit as a V5 keeping the
local variable - lemme know.
Cheers,
Guilherme
On Fri, 10 Feb 2023 17:35:10 -0300 "Guilherme G. Piccoli" <[email protected]> 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.
>
> Fix it by checking the NMI backtrace bit in the panic_print prior to
> the CPU disabling function.
>
> ...
>
> 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.
Only two sites change crash_kexec_post_notifiers, in
arch/powerpc/kernel/fadump.c and drivers/hv/hv_common.c. Yes, it's
very unlikely that this will be altered while panic() is running and
the consequences will be slight anyway.
But formally, we shouldn't do this, especially in a -stable
backportable patch. So please, let's have the minimal bugfix for now
and we can look at removing that local at a later time?
On 26/02/2023 02:44, Andrew Morton wrote:
> On Fri, 10 Feb 2023 17:35:10 -0300 "Guilherme G. Piccoli" <[email protected]> wrote:
> [...]
>> 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.
>
> Only two sites change crash_kexec_post_notifiers, in
> arch/powerpc/kernel/fadump.c and drivers/hv/hv_common.c. Yes, it's
> very unlikely that this will be altered while panic() is running and
> the consequences will be slight anyway.
>
> But formally, we shouldn't do this, especially in a -stable
> backportable patch. So please, let's have the minimal bugfix for now
> and we can look at removing that local at a later time?
>
Thanks Andrew, I agree with you! I just sent a V5 with the bugfix alone,
not changing this local/global variable behavior.
Cheers,
Guilherme