2022-04-21 00:43:29

by John Ogness

Subject: [PATCH printk v3 05/15] 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 <[email protected]>
---
kernel/printk/printk.c | 28 ++++++++++++++++------------
1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e23357002648..7bb148a1debb 100644
--- 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)
--
2.30.2

2022-04-21 18:58:45

by Petr Mladek

Subject: Re: [PATCH printk v3 05/15] printk: wake waiters for safe and NMI contexts

On Wed 2022-04-20 01:52:27, John Ogness wrote:
> 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 <[email protected]>

Another great catch!

Reviewed-by: Petr Mladek <[email protected]>

Best Regards,
Petr