Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
From:   Sasha Levin <sashal@kernel.org>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
Cc:     Stephen Brennan <stephen.s.brennan@oracle.com>,
        Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Sasha Levin <sashal@kernel.org>, linux@roeck-us.net,
        gregkh@linuxfoundation.org
Subject: [PATCH AUTOSEL 5.16 33/59] printk: Drop console_sem during panic
Date:   Wed, 30 Mar 2022 07:48:05 -0400
Message-Id: <20220330114831.1670235-33-sashal@kernel.org>
In-Reply-To: <20220330114831.1670235-1-sashal@kernel.org>
References: <20220330114831.1670235-1-sashal@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

From: Stephen Brennan <stephen.s.brennan@oracle.com>

[ Upstream commit 8ebc476fd51e6c0fd3174ec1959a20ba99d4c5e5 ]

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>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220202171821.179394-5-stephen.s.brennan@oracle.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 f7c4c53a2c35..92eb7785e6e7 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2588,6 +2588,25 @@ 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?
  *
@@ -2736,6 +2755,10 @@ 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();
 	}
@@ -2753,7 +2776,7 @@ 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);
-- 
2.34.1