Commit 100bdef2 authored by John Ogness's avatar John Ogness Committed by Petr Mladek
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console: introduce wrappers to read/write console flags 



After switching to SRCU for console list iteration, some readers
will begin readings console->flags as a data race. Locklessly
reading console->flags provides a consistent value because there
is at most one CPU modifying console->flags and that CPU is
using only read-modify-write operations.

Introduce a wrapper for SRCU iterators to read console flags.
Introduce a matching wrapper to write to flags of registered
consoles. Writing to flags of registered consoles is synchronized
by the console_list_lock.

Signed-off-by: default avatarJohn Ogness <john.ogness@linutronix.de>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221116162152.193147-13-john.ogness@linutronix.de
parent 4dc64682

include/linux/console.h

+45 −0
Original line number Diff line number Diff line
@@ -183,6 +183,51 @@ extern void console_list_unlock(void) __releases(console_mutex); 


extern struct hlist_head console_list;



/**

 * console_srcu_read_flags - Locklessly read the console flags

 * @con:	struct console pointer of console to read flags from

 *

 * This function provides the necessary READ_ONCE() and data_race()

 * notation for locklessly reading the console flags. The READ_ONCE()

 * in this function matches the WRITE_ONCE() when @flags are modified

 * for registered consoles with console_srcu_write_flags().

 *

 * Only use this function to read console flags when locklessly

 * iterating the console list via srcu.

 *

 * Context: Any context.

 */

static inline short console_srcu_read_flags(const struct console *con)

{

	WARN_ON_ONCE(!console_srcu_read_lock_is_held());



	/*

	 * Locklessly reading console->flags provides a consistent

	 * read value because there is at most one CPU modifying

	 * console->flags and that CPU is using only read-modify-write

	 * operations to do so.

	 */

	return data_race(READ_ONCE(con->flags));

}



/**

 * console_srcu_write_flags - Write flags for a registered console

 * @con:	struct console pointer of console to write flags to

 * @flags:	new flags value to write

 *

 * Only use this function to write flags for registered consoles. It

 * requires holding the console_list_lock.

 *

 * Context: Any context.

 */

static inline void console_srcu_write_flags(struct console *con, short flags)

{

	lockdep_assert_console_list_lock_held();



	/* This matches the READ_ONCE() in console_srcu_read_flags(). */

	WRITE_ONCE(con->flags, flags);

}



/**

 * for_each_console_srcu() - Iterator over registered consoles

 * @con:	struct console pointer used as loop cursor

kernel/printk/printk.c

+5 −5
Original line number Diff line number Diff line
@@ -3069,7 +3069,7 @@ void console_stop(struct console *console) 
	__pr_flush(console, 1000, true);

	console_list_lock();

	console_lock();

	console->flags &= ~CON_ENABLED;

	console_srcu_write_flags(console, console->flags & ~CON_ENABLED);

	console_unlock();

	console_list_unlock();
@@ -3087,7 +3087,7 @@ void console_start(struct console *console) 
{

	console_list_lock();

	console_lock();

	console->flags |= CON_ENABLED;

	console_srcu_write_flags(console, console->flags | CON_ENABLED);

	console_unlock();

	console_list_unlock();

	__pr_flush(console, 1000, true);
@@ -3343,7 +3343,7 @@ void register_console(struct console *newcon) 


	} else if (newcon->flags & CON_CONSDEV) {

		/* Only the new head can have CON_CONSDEV set. */

		console_first()->flags &= ~CON_CONSDEV;

		console_srcu_write_flags(console_first(), console_first()->flags & ~CON_CONSDEV);

		hlist_add_head_rcu(&newcon->node, &console_list);



	} else {
@@ -3400,7 +3400,7 @@ static int unregister_console_locked(struct console *console) 
	console_lock();



	/* Disable it unconditionally */

	console->flags &= ~CON_ENABLED;

	console_srcu_write_flags(console, console->flags & ~CON_ENABLED);



	if (hlist_unhashed(&console->node)) {

		console_unlock();
@@ -3419,7 +3419,7 @@ static int unregister_console_locked(struct console *console) 
	 * console has any device attached. Oh well....

	 */

	if (!hlist_empty(&console_list) && console->flags & CON_CONSDEV)

		console_first()->flags |= CON_CONSDEV;

		console_srcu_write_flags(console_first(), console_first()->flags | CON_CONSDEV);



	console_unlock();