Commit 1f5d7830 authored by John Ogness's avatar John Ogness Committed by Petr Mladek
Browse files

printk: add missing memory barrier to wake_up_klogd() 



It is important that any new records are visible to preparing
waiters before the waker checks if the wait queue is empty.
Otherwise it is possible that:

- there are new records available
- the waker sees an empty wait queue and does not wake
- the preparing waiter sees no new records and begins to wait

This is exactly the problem that the function description of
waitqueue_active() warns about.

Use wq_has_sleeper() instead of waitqueue_active() because it
includes the necessary full memory barrier.

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/20220421212250.565456-4-john.ogness@linutronix.de
parent f5343321

kernel/printk/printk.c

+36 −3
Original line number Diff line number Diff line
@@ -746,8 +746,19 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf, 
			goto out;

		}



		/*

		 * Guarantee this task is visible on the waitqueue before

		 * checking the wake condition.

		 *

		 * The full memory barrier within set_current_state() of

		 * prepare_to_wait_event() pairs with the full memory barrier

		 * within wq_has_sleeper().

		 *

		 * This pairs with wake_up_klogd:A.

		 */

		ret = wait_event_interruptible(log_wait,

				prb_read_valid(prb, atomic64_read(&user->seq), r));

				prb_read_valid(prb,

					atomic64_read(&user->seq), r)); /* LMM(devkmsg_read:A) */

		if (ret)

			goto out;

	}
@@ -1513,7 +1524,18 @@ static int syslog_print(char __user *buf, int size) 
		seq = syslog_seq;



		mutex_unlock(&syslog_lock);

		len = wait_event_interruptible(log_wait, prb_read_valid(prb, seq, NULL));

		/*

		 * Guarantee this task is visible on the waitqueue before

		 * checking the wake condition.

		 *

		 * The full memory barrier within set_current_state() of

		 * prepare_to_wait_event() pairs with the full memory barrier

		 * within wq_has_sleeper().

		 *

		 * This pairs with wake_up_klogd:A.

		 */

		len = wait_event_interruptible(log_wait,

				prb_read_valid(prb, seq, NULL)); /* LMM(syslog_print:A) */

		mutex_lock(&syslog_lock);



		if (len)
@@ -3316,7 +3338,18 @@ void wake_up_klogd(void) 
		return;



	preempt_disable();

	if (waitqueue_active(&log_wait)) {

	/*

	 * Guarantee any new records can be seen by tasks preparing to wait

	 * before this context checks if the wait queue is empty.

	 *

	 * The full memory barrier within wq_has_sleeper() pairs with the full

	 * memory barrier within set_current_state() of

	 * prepare_to_wait_event(), which is called after ___wait_event() adds

	 * the waiter but before it has checked the wait condition.

	 *

	 * This pairs with devkmsg_read:A and syslog_print:A.

	 */

	if (wq_has_sleeper(&log_wait)) { /* LMM(wake_up_klogd:A) */

		this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);

		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));

	}