[PATCH printk v2 4/4] printk: Ignore waiter on panic

[John Ogness <john.ogness@linutronix.de>]

Commit d51507098ff91 ("printk: disable optimistic spin during
panic") added checks to avoid becoming a console waiter if a
panic is in progress. However, the transition to panic can occur
while there is already a waiter. If the panic occurred in a
context that does not support printing from the printk() caller
context, the waiter CPU may become stopped while still stored as
@console_waiter. Then when console_flush_on_panic() is called,
the panic CPU will see @console_waiter and handover to the
stopped CPU.

Here a simple example:

CPU0                                CPU1
----                                ----
console_lock_spinning_enable()
                                    console_trylock_spinning()
                                      [set as console waiter]
NMI: panic()
panic_other_cpus_shutdown()
                                    [stopped as console waiter]
console_flush_on_panic()
  console_lock_spinning_enable()
    [print 1 record]
  console_lock_spinning_disable_and_check()
    [handover to stopped CPU1]

This results in panic() not flushing the panic messages.

Fix this by ignoring any console waiter if the panic CPU is
printing.

Fixes: dbdda842fe96 ("printk: Add console owner and waiter logic to load balance console writes")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/printk/printk.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 56d9b4acbbf2..cd6493f12970 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1904,7 +1904,8 @@ static int console_lock_spinning_disable_and_check(int cookie)
 	console_owner = NULL;
 	raw_spin_unlock(&console_owner_lock);
 
-	if (!waiter) {
+	/* Waiters are ignored by the panic CPU. */
+	if (!waiter || this_cpu_in_panic()) {
 		spin_release(&console_owner_dep_map, _THIS_IP_);
 		return 0;
 	}
-- 
2.39.2