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From:   Stephen Brennan <stephen.s.brennan@oracle.com>
To:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Petr Mladek <pmladek@suse.com>
Cc:     linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        John Ogness <john.ogness@linutronix.de>,
        Stephen Brennan <stephen.s.brennan@oracle.com>
Subject: [PATCH v4 4/4] printk: Drop console_sem during panic
Date:   Wed,  2 Feb 2022 09:18:21 -0800
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If another CPU is in panic, we are about to be halted. Try to gracefully
abandon the console_sem, leaving it free for the panic CPU to grab.

Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
---

Notes:
    v2: Factor check out to a helper, and check at the end of
    console_unlock() to prevent retry as well.

 kernel/printk/printk.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 736f00d33087..a174c4b2d156 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2598,6 +2598,25 @@ static int have_callable_console(void)
 	return 0;
 }
 
+/*
+ * Return true when this CPU should unlock console_sem without pushing all
+ * messages to the console. This reduces the chance that the console is
+ * locked when the panic CPU tries to use it.
+ */
+static bool abandon_console_lock_in_panic(void)
+{
+	if (!panic_in_progress())
+		return false;
+
+	/*
+	 * We can use raw_smp_processor_id() here because it is impossible for
+	 * the task to be migrated to the panic_cpu, or away from it. If
+	 * panic_cpu has already been set, and we're not currently executing on
+	 * that CPU, then we never will be.
+	 */
+	return atomic_read(&panic_cpu) != raw_smp_processor_id();
+}
+
 /*
  * Can we actually use the console at this time on this cpu?
  *
@@ -2746,6 +2765,10 @@ void console_unlock(void)
 		if (handover)
 			return;
 
+		/* Allow panic_cpu to take over the consoles safely */
+		if (abandon_console_lock_in_panic())
+			break;
+
 		if (do_cond_resched)
 			cond_resched();
 	}
@@ -2763,7 +2786,7 @@ void console_unlock(void)
 	 * flush, no worries.
 	 */
 	retry = prb_read_valid(prb, next_seq, NULL);
-	if (retry && console_trylock())
+	if (retry && !abandon_console_lock_in_panic() && console_trylock())
 		goto again;
 }
 EXPORT_SYMBOL(console_unlock);
-- 
2.30.2