printk: wake waiters for safe and NMI contexts

When printk() is called from safe or NMI contexts, it will directly
store the record (vprintk_store()) and then defer the console output.
However, defer_console_output() only causes console printing and does
not wake any waiters of new records.

Wake waiters from defer_console_output() so that they also are aware
of the new records from safe and NMI contexts.

Fixes: 03fc7f9c99c1 ("printk/nmi: Prevent deadlock when accessing the main log buffer in NMI")
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/20220421212250.565456-6-john.ogness@linutronix.de

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e233570..7bb148a 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -754,7 +754,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
                 * prepare_to_wait_event() pairs with the full memory barrier
                 * within wq_has_sleeper().
                 *
-                * This pairs with wake_up_klogd:A.
+                * This pairs with __wake_up_klogd:A.
                 */
                ret = wait_event_interruptible(log_wait,
                                prb_read_valid(prb,
@@ -1532,7 +1532,7 @@ static int syslog_print(char __user *buf, int size)
                 * prepare_to_wait_event() pairs with the full memory barrier
                 * within wq_has_sleeper().
                 *
-                * This pairs with wake_up_klogd:A.
+                * This pairs with __wake_up_klogd:A.
                 */
                len = wait_event_interruptible(log_wait,
                                prb_read_valid(prb, seq, NULL)); /* LMM(syslog_print:A) */
@@ -3332,7 +3332,7 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work)
 static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) =
        IRQ_WORK_INIT_LAZY(wake_up_klogd_work_func);
 
-void wake_up_klogd(void)
+static void __wake_up_klogd(int val)
 {
        if (!printk_percpu_data_ready())
                return;
@@ -3349,22 +3349,26 @@ void wake_up_klogd(void)
         *
         * 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);
+       if (wq_has_sleeper(&log_wait) || /* LMM(__wake_up_klogd:A) */
+           (val & PRINTK_PENDING_OUTPUT)) {
+               this_cpu_or(printk_pending, val);
                irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
        }
        preempt_enable();
 }
 
-void defer_console_output(void)
+void wake_up_klogd(void)
 {
-       if (!printk_percpu_data_ready())
-               return;
+       __wake_up_klogd(PRINTK_PENDING_WAKEUP);
+}
 
-       preempt_disable();
-       this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
-       irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
-       preempt_enable();
+void defer_console_output(void)
+{
+       /*
+        * New messages may have been added directly to the ringbuffer
+        * using vprintk_store(), so wake any waiters as well.
+        */
+       __wake_up_klogd(PRINTK_PENDING_WAKEUP | PRINTK_PENDING_OUTPUT);
 }
 
 void printk_trigger_flush(void)