Commit fc956ae0 authored by John Ogness's avatar John Ogness Committed by Petr Mladek
Browse files

printk: console_flush_all: use srcu console list iterator 



Guarantee safe iteration of the console list by using SRCU.

Note that in the case of a handover, the SRCU read lock is also
released. This is documented in the function description and as
comments in the code. It is a bit tricky, but this preserves the
lockdep lock ordering for the context handing over the
console_lock:

  console_lock()
  | mutex_acquire(&console_lock_dep_map)       <-- console lock
  |
  console_unlock()
  | console_flush_all()
  | | srcu_read_lock(&console_srcu)            <-- srcu lock
  | | console_emit_next_record()
  | | | console_lock_spinning_disable_and_check()
  | | | | srcu_read_unlock(&console_srcu)      <-- srcu unlock
  | | | | mutex_release(&console_lock_dep_map) <-- console unlock

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/20221116162152.193147-16-john.ogness@linutronix.de
parent b8ef04be

kernel/printk/printk.c

+35 −15
Original line number Diff line number Diff line
@@ -1894,13 +1894,13 @@ static void console_lock_spinning_enable(void) 
 * safe to start busy waiting for the lock. Second, it checks if

 * there is a busy waiter and passes the lock rights to her.

 *

 * Important: Callers lose the lock if there was a busy waiter.

 *	They must not touch items synchronized by console_lock

 *	in this case.

 * Important: Callers lose both the console_lock and the SRCU read lock if

 *	there was a busy waiter. They must not touch items synchronized by

 *	console_lock or SRCU read lock in this case.

 *

 * Return: 1 if the lock rights were passed, 0 otherwise.

 */

static int console_lock_spinning_disable_and_check(void)

static int console_lock_spinning_disable_and_check(int cookie)

{

	int waiter;
@@ -1919,6 +1919,12 @@ static int console_lock_spinning_disable_and_check(void) 


	spin_release(&console_owner_dep_map, _THIS_IP_);



	/*

	 * Preserve lockdep lock ordering. Release the SRCU read lock before

	 * releasing the console_lock.

	 */

	console_srcu_read_unlock(cookie);



	/*

	 * Hand off console_lock to waiter. The waiter will perform

	 * the up(). After this, the waiter is the console_lock owner.
@@ -2402,7 +2408,7 @@ static ssize_t msg_print_ext_body(char *buf, size_t size, 
				  char *text, size_t text_len,

				  struct dev_printk_info *dev_info) { return 0; }

static void console_lock_spinning_enable(void) { }

static int console_lock_spinning_disable_and_check(void) { return 0; }

static int console_lock_spinning_disable_and_check(int cookie) { return 0; }

static void call_console_driver(struct console *con, const char *text, size_t len,

				char *dropped_text)

{
@@ -2744,16 +2750,18 @@ static void __console_unlock(void) 
 * DROPPED_TEXT_MAX. Otherwise @dropped_text must be NULL.

 *

 * @handover will be set to true if a printk waiter has taken over the

 * console_lock, in which case the caller is no longer holding the

 * console_lock. Otherwise it is set to false.

 * console_lock, in which case the caller is no longer holding both the

 * console_lock and the SRCU read lock. Otherwise it is set to false.

 *

 * @cookie is the cookie from the SRCU read lock.

 *

 * Returns false if the given console has no next record to print, otherwise

 * true.

 *

 * Requires the console_lock.

 * Requires the console_lock and the SRCU read lock.

 */

static bool console_emit_next_record(struct console *con, char *text, char *ext_text,

				     char *dropped_text, bool *handover)

				     char *dropped_text, bool *handover, int cookie)

{

	static int panic_console_dropped;

	struct printk_info info;
@@ -2813,7 +2821,7 @@ static bool console_emit_next_record(struct console *con, char *text, char *ext_ 


	con->seq++;



	*handover = console_lock_spinning_disable_and_check();

	*handover = console_lock_spinning_disable_and_check(cookie);

	printk_safe_exit_irqrestore(flags);

skip:

	return true;
@@ -2850,6 +2858,7 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove 
	bool any_usable = false;

	struct console *con;

	bool any_progress;

	int cookie;



	*next_seq = 0;

	*handover = false;
@@ -2857,23 +2866,29 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove 
	do {

		any_progress = false;



		for_each_console(con) {

		cookie = console_srcu_read_lock();

		for_each_console_srcu(con) {

			bool progress;



			if (!console_is_usable(con))

				continue;

			any_usable = true;



			if (con->flags & CON_EXTENDED) {

			if (console_srcu_read_flags(con) & CON_EXTENDED) {

				/* Extended consoles do not print "dropped messages". */

				progress = console_emit_next_record(con, &text[0],

								    &ext_text[0], NULL,

								    handover);

								    handover, cookie);

			} else {

				progress = console_emit_next_record(con, &text[0],

								    NULL, &dropped_text[0],

								    handover);

								    handover, cookie);

			}



			/*

			 * If a handover has occurred, the SRCU read lock

			 * is already released.

			 */

			if (*handover)

				return false;
@@ -2887,14 +2902,19 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove 


			/* Allow panic_cpu to take over the consoles safely. */

			if (abandon_console_lock_in_panic())

				return false;

				goto abandon;



			if (do_cond_resched)

				cond_resched();

		}

		console_srcu_read_unlock(cookie);

	} while (any_progress);



	return any_usable;



abandon:

	console_srcu_read_unlock(cookie);

	return false;

}



/**