[PATCH printk v2 08/12] printk: add pr_flush()

John Ogness
2022-04-05T20:57:09Z

Provide a might-sleep function to allow waiting for console printers
to catch up to the latest logged message.

Use pr_flush() whenever it is desirable to get buffered messages
printed before continuing: suspend_console(), resume_console(),
console_stop(), console_start(), console_unblank().

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
include/linux/printk.h | 7 ++++
kernel/printk/printk.c | 83 ++++++++++++++++++++++++++++++++++++++++++
2 files changed, 90 insertions(+)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index b70a42f94031..091fba7283e1 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -170,6 +170,8 @@ extern void __printk_safe_exit(void);
#define printk_deferred_enter __printk_safe_enter
#define printk_deferred_exit __printk_safe_exit

+extern bool pr_flush(int timeout_ms, bool reset_on_progress);
+
/*
* Please don't use printk_ratelimit(), because it shares ratelimiting state
* with all other unrelated printk_ratelimit() callsites. Instead use
@@ -220,6 +222,11 @@ static inline void printk_deferred_exit(void)
{
}

+static inline bool pr_flush(int timeout_ms, bool reset_on_progress)
+{
+ return true;
+}
+
static inline int printk_ratelimit(void)
{
return 0;
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 06f0b29d909d..d0369afaf365 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2274,6 +2274,8 @@ asmlinkage __visible int _printk(const char *fmt, ...)
}
EXPORT_SYMBOL(_printk);

+static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress);
+
#else /* CONFIG_PRINTK */

#define CONSOLE_LOG_MAX 0
@@ -2306,6 +2308,7 @@ static void call_console_driver(struct console *con, const char *text, size_t le
{
}
static bool suppress_message_printing(int level) { return false; }
+static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress) { return true; }

#endif /* CONFIG_PRINTK */

@@ -2493,6 +2496,7 @@ void suspend_console(void)
if (!console_suspend_enabled)
return;
pr_info("Suspending console(s) (use no_console_suspend to debug)\n");
+ pr_flush(1000, true);
console_lock();
console_suspended = 1;
up_console_sem();
@@ -2505,6 +2509,7 @@ void resume_console(void)
down_console_sem();
console_suspended = 0;
console_unlock();
+ pr_flush(1000, true);
}

/**
@@ -2882,6 +2887,9 @@ void console_unblank(void)
if ((c->flags & CON_ENABLED) && c->unblank)
c->unblank();
console_unlock();
+
+ if (!oops_in_progress)
+ pr_flush(1000, true);
}

/**
@@ -2940,6 +2948,7 @@ struct tty_driver *console_device(int *index)
*/
void console_stop(struct console *console)
{
+ __pr_flush(console, 1000, true);
console_lock();
console->flags &= ~CON_ENABLED;
console_unlock();
@@ -2951,6 +2960,7 @@ void console_start(struct console *console)
console_lock();
console->flags |= CON_ENABLED;
console_unlock();
+ __pr_flush(console, 1000, true);
}
EXPORT_SYMBOL(console_start);

@@ -3321,6 +3331,79 @@ static int __init printk_late_init(void)
late_initcall(printk_late_init);

#if defined CONFIG_PRINTK
+/* If @con is specified, only wait for that console. Otherwise wait for all. */
+static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress)
+{
+ int remaining = timeout_ms;
+ struct console *c;
+ u64 last_diff = 0;
+ u64 printk_seq;
+ u64 diff;
+ u64 seq;
+
+ might_sleep();
+
+ seq = prb_next_seq(prb);
+
+ for (;;) {
+ diff = 0;
+
+ console_lock();
+ for_each_console(c) {
+ if (con && con != c)
+ continue;
+ if (!console_is_usable(c))
+ continue;
+ printk_seq = c->seq;
+ if (printk_seq < seq)
+ diff += seq - printk_seq;
+ }
+ console_unlock();
+
+ if (diff != last_diff && reset_on_progress)
+ remaining = timeout_ms;
+
+ if (diff == 0 || remaining == 0)
+ break;
+
+ if (remaining < 0) {
+ /* no timeout limit */
+ msleep(100);
+ } else if (remaining < 100) {
+ msleep(remaining);
+ remaining = 0;
+ } else {
+ msleep(100);
+ remaining -= 100;
+ }
+
+ last_diff = diff;
+ }
+
+ return (diff == 0);
+}
+
+/**
+ * pr_flush() - Wait for printing threads to catch up.
+ *
+ * @timeout_ms: The maximum time (in ms) to wait.
+ * @reset_on_progress: Reset the timeout if forward progress is seen.
+ *
+ * A value of 0 for @timeout_ms means no waiting will occur. A value of -1
+ * represents infinite waiting.
+ *
+ * If @reset_on_progress is true, the timeout will be reset whenever any
+ * printer has been seen to make some forward progress.
+ *
+ * Context: Process context. May sleep while acquiring console lock.
+ * Return: true if all enabled printers are caught up.
+ */
+bool pr_flush(int timeout_ms, bool reset_on_progress)
+{
+ return __pr_flush(NULL, timeout_ms, reset_on_progress);
+}
+EXPORT_SYMBOL(pr_flush);
+
/*
* Delayed printk version, for scheduler-internal messages:
*/
--
2.30.2