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Date: Thu, 13 Jan 2022 18:39:25 +0100
From: Petr Mladek <pmladek@...e.com>
To: Stephen Brennan <stephen.s.brennan@...cle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Sergey Senozhatsky <senozhatsky@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>,
John Ogness <john.ogness@...utronix.de>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] panic: disable optimistic spin after halting CPUs
On Wed 2022-01-12 16:54:25, Stephen Brennan wrote:
> A CPU executing with console lock spinning enabled might be halted
> during a panic. Before console_flush_on_panic(), it's possible for
> console_trylock() to attempt optimistic spinning, deadlocking the panic
> CPU:
>
> CPU 0 (panic CPU) CPU 1
> ----------------- ------
> printk() {
> vprintk_func() {
> vprintk_default() {
> vprintk_emit() {
> console_unlock() {
> console_lock_spinning_enable();
> ... printing to console ...
> panic() {
> crash_smp_send_stop() {
> NMI -------------------> HALT
> }
> atomic_notifier_call_chain() {
> printk() {
> ...
> console_trylock_spinnning() {
> // optimistic spin infinitely
I see.
> This hang during panic can be induced when a kdump kernel is loaded, and
> crash_kexec_post_notifiers=1 is present on the kernel command line. The
> following script which concurrently writes to /dev/kmsg, and triggers a
> panic, can result in this hang:
>
> #!/bin/bash
> date
> # 991 chars (based on log buffer size):
> chars="$(printf 'a%.0s' {1..991})"
> while :; do
> echo $chars > /dev/kmsg
> done &
> echo c > /proc/sysrq-trigger &
> date
> exit
>
> Below are the hang rates for the above test case, based on v5.16-rc8
> before and after this patch:
>
> before: 197 hangs / 340 trials - 57.9%
> after : 0 hangs / 245 trials - 0.0%
>
> Fixes: dbdda842fe96 ("printk: Add console owner and waiter logic to load balance console writes")
>
> Signed-off-by: Stephen Brennan <stephen.s.brennan@...cle.com>
> Reviewed-by: Junxiao Bi <junxiao.bi@...cle.com>
> ---
> include/linux/console.h | 1 +
> kernel/panic.c | 7 +++++++
> kernel/printk/printk.c | 17 +++++++++++++++++
> 3 files changed, 25 insertions(+)
>
> diff --git a/include/linux/console.h b/include/linux/console.h
> index a97f277cfdfa..4eeb46927d96 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -179,6 +179,7 @@ extern void console_unlock(void);
> extern void console_conditional_schedule(void);
> extern void console_unblank(void);
> extern void console_flush_on_panic(enum con_flush_mode mode);
> +extern void console_lock_spinning_disable_on_panic(void);
> extern struct tty_driver *console_device(int *);
> extern void console_stop(struct console *);
> extern void console_start(struct console *);
> diff --git a/kernel/panic.c b/kernel/panic.c
> index cefd7d82366f..a9340e580b20 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -265,6 +265,13 @@ void panic(const char *fmt, ...)
> crash_smp_send_stop();
> }
>
> + /*
> + * Now that we've halted other CPUs, disable optimistic spinning in
> + * printk(). This avoids deadlocking in console_trylock(), until we call
> + * console_flush_on_panic().
> + */
> + console_lock_spinning_disable_on_panic();
The proposed implementation causes that the panicing CPU takes over
console_lock even when the current owner is in the middle of
call_console_drivers().
It means that calling console drivers from another CPU is not
completely safe. This is why console_flush_on_panic() is called
at end when the console is the only way to see the messages.
We should not do this before kmsg_dump() and __crash_kexec()
that allow to see the messages a "more safe" way.
I suggest to disable the spinning when panic is in progress. I mean
something like:
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1842,6 +1842,10 @@ static int console_trylock_spinning(void)
if (console_trylock())
return 1;
+ /* Spinning is not safe when the system is stopped */
+ if (read_atomic(&panic_cpu) != PANIC_CPU_INVALID)
+ return 0;
+
printk_safe_enter_irqsave(flags);
raw_spin_lock(&console_owner_lock);
It would be also great when the current owner releases console_lock
so that the panicing CPU could take over it.
I think about the following. Well, I am not sure if it would help
in the real life because other CPUs are stopped quite early in panic().
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2646,13 +2650,18 @@ void console_unlock(void)
for (;;) {
size_t ext_len = 0;
- int handover;
+ int handover, pcpu;
size_t len;
skip:
if (!prb_read_valid(prb, console_seq, &r))
break;
+ /* Allow the panic_cpu to take over consoles a safe way. */
+ pcpu = read_atomic(&panic_cpu);
+ if (pcpu != PANIC_CPU_INVALID && pcpu != smp_processor_id())
+ break;
+
if (console_seq != r.info->seq) {
console_dropped += r.info->seq - console_seq;
console_seq = r.info->seq;
Note that the above code is not even compile tested. panic_cpu is
local variable in panic.c. If we go this way than I would define
some helpers:
bool panic_in_progress(void)
{
return read_atomic(&panic_cpu) != PANIC_CPU_INVALID;
}
bool panic_in_progress_on_other_cpu(void)
{
int pcpu = read_atomic(&panic_cpu);
if (pcpu == PANIC_CPU_INVALID)
return false;
if (pcpu == smp_processor_id())
return false;
return true;
}
Best Regards,
Petr
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