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: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220421212250.565456-4-john.ogness@linutronix.de

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 13a1eeb..f817dfb 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -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));
        }