[PATCH 4/4] printk: Drop console_sem during panic

[Stephen Brennan]

If another CPU is in panic, we are about to be halted. Try to gracefully
drop console_sem and allow the panic CPU to grab it easily.

Suggested-by: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
---
 kernel/printk/printk.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ca253ac07615..c2dc8ebd9509 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2668,7 +2668,7 @@ void console_unlock(void)
 
 	for (;;) {
 		size_t ext_len = 0;
-		int handover;
+		int handover, pcpu;
 		size_t len;
 
 skip:
@@ -2739,6 +2739,12 @@ void console_unlock(void)
 		if (handover)
 			return;
 
+		/* Allow panic_cpu to take over the consoles safely */
+		pcpu = atomic_read(&panic_cpu);
+		if (unlikely(pcpu != PANIC_CPU_INVALID &&
+		    pcpu != raw_smp_processor_id()))
+			break;
+
 		if (do_cond_resched)
 			cond_resched();
 	}
-- 
2.30.2