[PATCH printk v1 13/13] console: introduce CON_MIGHT_SLEEP for vt

[John Ogness <john.ogness@linutronix.de>]

Deadlocks and the framebuffer console have been a recurring issue
that is getting worse. Daniel Vetter suggested [0] that
fbcon->write() should no longer be called from an atomic context.

Introduce a new console flag CON_MIGHT_SLEEP for a console driver to
specify that it is only called from sleepable contexts. Set the
fbcon to use this new flag.

[0] https://lore.kernel.org/all/YYuS1uNhxWOEX1Ci@phenom.ffwll.local

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 drivers/tty/vt/vt.c     | 2 +-
 include/linux/console.h | 1 +
 kernel/printk/printk.c  | 2 ++
 3 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 7359c3e80d63..ab4712cc9327 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3161,7 +3161,7 @@ static struct console vt_console_driver = {
 	.write		= vt_console_print,
 	.device		= vt_console_device,
 	.unblank	= unblank_screen,
-	.flags		= CON_PRINTBUFFER,
+	.flags		= CON_PRINTBUFFER|CON_MIGHT_SLEEP,
 	.index		= -1,
 };
 #endif
diff --git a/include/linux/console.h b/include/linux/console.h
index c51c7f5507a5..ea52c56b3ff8 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -138,6 +138,7 @@ static inline int con_debug_leave(void)
 #define CON_BRL		(32) /* Used for a braille device */
 #define CON_EXTENDED	(64) /* Use the extended output format a la /dev/kmsg */
 #define CON_PAUSED	(128) /* Sleep while console is locked */
+#define CON_MIGHT_SLEEP	(256) /* Can only be called from sleepable context */
 
 struct console {
 	char	name[16];
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 086155578f10..b92ef67a5aa2 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2836,6 +2836,8 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 
 			if (!console_is_usable(con))
 				continue;
+			if ((con->flags & CON_MIGHT_SLEEP) && !do_cond_resched)
+				continue;
 			any_usable = true;
 
 			if (con->flags & CON_EXTENDED) {
-- 
2.30.2