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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,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: [PATCH printk v2 04/38] printk: introduce console_is_enabled() wrapper
Date:   Wed, 19 Oct 2022 17:01:26 +0206
Message-Id: <20221019145600.1282823-5-john.ogness@linutronix.de>
In-Reply-To: <20221019145600.1282823-1-john.ogness@linutronix.de>
References: <20221019145600.1282823-1-john.ogness@linutronix.de>
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After switching to SRCU for console list iteration, some readers
will begin accessing console->flags as a data race. This is safe
because there is at most one CPU modifying console->flags and
using rmw operations.

The primary reason for readers to access console->flags is to
check if the console is enabled. Introduce console_is_enabled()
to mark such access as a data race.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 include/linux/console.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/include/linux/console.h b/include/linux/console.h
index cff86cc615f8..60195cd086dc 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -172,6 +172,26 @@ extern void console_srcu_read_unlock(int cookie);
 
 extern struct hlist_head console_list;
 
+/**
+ * console_is_enabled - Check if the console is enabled
+ * @con:	struct console pointer of console to check
+ *
+ * This should be used instead of manually testing for the CON_ENABLED
+ * bit in the console->flags.
+ *
+ * Context: Any context.
+ */
+static inline bool console_is_enabled(const struct console *con)
+{
+	/*
+	 * If SRCU is used, reading of console->flags can be a data
+	 * race. However, this is safe because there is at most one
+	 * CPU modifying console->flags and it is using only
+	 * read-modify-write operations to do so.
+	 */
+	return (data_race(READ_ONCE(con->flags)) & CON_ENABLED);
+}
+
 /**
  * for_each_console_srcu() - Iterator over registered consoles
  * @con:	struct console pointer used as loop cursor
-- 
2.30.2