[PATCH v2] printk: disable optimistic spin during panic

[Stephen Brennan]

A CPU executing with console lock spinning enabled might be halted
during a panic. Before the panicking CPU calls console_flush_on_panic(),
it may call console_trylock(), which attempts to optimistically spin,
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

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

To avoid this deadlock, ensure that console_trylock_spinning() does not
allow spinning once a panic has begun. Below are the hang rates for the
above test case, based on v5.16-rc8 before and after this patch:

before:  776 hangs / 1484 trials - 52.3%
after :    0 hangs /  102 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@xxxxxxxxxx>
---

Note: I continue to run my tests past 100 trials, but at this point I'm
confident enough in the fix to send the patch.

As Petr suggested I will send another patch making the non-panic CPUs
bail out of console_unlock(), but I wanted to get this one first, since
it is implemented and ready now.

 kernel/printk/printk.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 57b132b658e1..612ed895b967 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1843,6 +1843,16 @@ static int console_trylock_spinning(void)
 	if (console_trylock())
 		return 1;
 
+	/*
+	 * It's unsafe for the panic CPU to spin, since the CPU holding the
+	 * console_sem may have already been halted. However, if a panic has
+	 * started, but we're not the panic CPU, we still spin. This protects
+	 * the panic CPU from needing to write out messages buffered by other
+	 * CPUs in console_unlock().
+	 */
+	if (atomic_read(&panic_cpu) == raw_smp_processor_id())
+		return 0;
+
 	printk_safe_enter_irqsave(flags);
 
 	raw_spin_lock(&console_owner_lock);
-- 
2.30.2