Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   John Ogness <john.ogness@linutronix.de>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org
Subject: [PATCH printk v4 18/39] printk: console_flush_on_panic: use srcu console list iterator
Date:   Mon, 14 Nov 2022 17:35:11 +0106
Message-Id: <20221114162932.141883-19-john.ogness@linutronix.de>
In-Reply-To: <20221114162932.141883-1-john.ogness@linutronix.de>
References: <20221114162932.141883-1-john.ogness@linutronix.de>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

With SRCU it is now safe to traverse the console list, even if
the console_trylock() failed. However, overwriting console->seq
when console_trylock() failed is still an issue.

Switch to SRCU iteration and document remaining issue with
console->seq.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/printk.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 7304cbcd6dd4..f536233d8234 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3049,21 +3049,22 @@ void console_flush_on_panic(enum con_flush_mode mode)
 	console_may_schedule = 0;
 
 	if (mode == CONSOLE_REPLAY_ALL) {
-		struct hlist_node *tmp;
 		struct console *c;
+		int cookie;
 		u64 seq;
 
 		seq = prb_first_valid_seq(prb);
-		/*
-		 * This cannot use for_each_console() because it's not established
-		 * that the current context has console locked and neither there is
-		 * a guarantee that there is no concurrency in that case.
-		 *
-		 * Open code it for documentation purposes and pretend that
-		 * it works.
-		 */
-		hlist_for_each_entry_safe(c, tmp, &console_list, node)
+
+		cookie = console_srcu_read_lock();
+		for_each_console_srcu(c) {
+			/*
+			 * If the above console_trylock() failed, this is an
+			 * unsynchronized assignment. But in that case, the
+			 * kernel is in "hope and pray" mode anyway.
+			 */
 			c->seq = seq;
+		}
+		console_srcu_read_unlock(cookie);
 	}
 	console_unlock();
 }
-- 
2.30.2