Re: [PATCH V3 11/11] panic: Fixes the panic_print NMI backtrace setting
On 19/08/2022 19:17, 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.
>
> 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: Feng Tang
> Cc: Petr Mladek
> Signed-off-by: Guilherme G. Piccoli
>
> ---
>
> V3:
> - No changes.
>
> V2:
> - new patch, there was no V1 of this one.
>
> Hi folks, thanks upfront for reviews. This is a new patch, fixing an issue
> I found in my tests, so I shoved it into this fixes series.
>
> 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
>
>
Hi folks, bi-monthly ping - apologies for the noise heh
Is there anything suggested so we can get this fix merged in 6.2? Any
suggestions / reviews are much appreciated.
Tnx in advance,
Guilherme
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Re: [PATCH V3 11/11] panic: Fixes the panic_print NMI backtrace setting
On 19/08/2022 19:17, 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.
>
> 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: Feng Tang
> Cc: Petr Mladek
> Signed-off-by: Guilherme G. Piccoli
>
> ---
>
> V3:
> - No changes.
>
> V2:
> - new patch, there was no V1 of this one.
>
> Hi folks, thanks upfront for reviews. This is a new patch, fixing an issue
> I found in my tests, so I shoved it into this fixes series.
>
> 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(-)
> [...]
Hi Andrew, sorry for the ping.
Does the patch makes sense for you? Any comments are much appreciated!
Tnx in advance,
Guilherme
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[PATCH V3 11/11] panic: Fixes the panic_print NMI backtrace setting
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: Feng Tang
Cc: Petr Mladek
Signed-off-by: Guilherme G. Piccoli
---
V3:
- No changes.
V2:
- new patch, there was no V1 of this one.
Hi folks, thanks upfront for reviews. This is a new patch, fixing an issue
I found in my tests, so I shoved it into this fixes series.
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 c6eb8f8db0c0..b025a8f21c17 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -180,9 +180,6 @@ 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();
@@ -199,6 +196,30 @@ static void panic_print_sys_info(bool console_flush)
ftrace_dump(DUMP_ALL);
}
+/*
+ * 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
@@ -214,7 +235,6 @@ 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) {
/*
@@ -289,23 +309,10 @@ 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
@@ -326,7 +333,7 @@ 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);
#ifdef CONFIG_VT
--
2.37.2
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